about 1986 R.C. Bradlee published a Xeroxed page book “Programming Autocad” which featured the Autolisp command set as of autocad version 2.5. Selection Sets had just been added. Entity names could be retrieved and properties modified but new entitities had to be added with the Command function (command “Line” pnt1 …). This would be a good starter section for learning the language, not overwhelming but not trivial. Some or most of the links seem to paste in from the Autodesk 2017 help reference. I rearranged and edited the entries into my own idea of fuzzy categories. A disclaimer – this is not an official list of Lisp 2.5 but just my reconstruction. Some functions i left out either inadvertently, or i assume they are obsolete, or simply i dont see a need for them.
Descriptions below all belong to Autodesk. Here is there notice cut / pasted from their help page.
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Math and booleans are first and easiest to master.
+ (add)  (+ [number number …])  Returns the sum of all numbers 
– (subtract)  ( [number number …])  Subtracts the second and following numbers from the first and returns the difference 
* (multiply)  (* [number number …])  Returns the product of all numbers 
/ (divide)  (/ [number number …])  Divides the first number by the product of the remaining numbers and returns the quotient 
1+ (increment)  (1+ number)  Returns the argument increased by 1 (incremented) 
1 (decrement)  (1 number)  Returns the argument reduced by 1 (decremented) 
abs  (abs number)  Returns the absolute value of the argument 
atan  (atan num1 [num2])  Returns the arctangent of a number in radians 
cos  (cos ang)  Returns the cosine of an angle expressed in radians 
exp  (exp number)  Returns the constant e (a real) raised to a specified power (the natural antilog) 
expt  (expt base power)  Returns a number raised to a specified power 
fix  (fix number)  Returns the conversion of a real into the nearest smaller integer 
float  (float number)  Returns the conversion of a number into a real 
gcd  (gcd int1 int2)  Returns the greatest common denominator of two integers 
log  (log number)  Returns the natural log of a number as a real 
max  (max [number number …])  Returns the largest of the numbers given 
min  (min [number number …])  Returns the smallest of the numbers given 
rem  (rem [num1 num2 …])  Divides the first number by the second, and returns the remainder 
sin  (sin ang)  Returns the sine of an angle as a real expressed in radians 
sqrt  (sqrt number)  Returns the square root of a number as a real 
~ (bitwise NOT)  (~ int)  Returns the bitwise NOT (1’s complement) of the argument 
logand  (logand [int int …])  Returns the result of the logical bitwise AND of a list of integers 
logior  (logior [int int …])  Returns the result of the logical bitwise inclusive OR of a list of integers 
lsh  (lsh [int numbits])  Returns the logical bitwise shift of an integer by a specified number of bits 
atom  (atom item)  Verifies that an item is an atom 
boundp  (boundp sym)  Verifies whether a value is bound to a symbol 
listp  (listp item)  Verifies that an item is a list 
not  (not item)  Verifies that an item evaluates to nil 
null  (null item)  Verifies that an item is bound to nil 
numberp  (numberp item)  Verifies that an item is a real or an integer 
minusp  (minusp number)  Verifies that a number is negative 
zerop  (zerop number)  Verifies that a number evaluates to zero 
= (equal to)  (= numstr [numstr …])  Returns T if all arguments are numerically equal, and returns nil otherwise 
/= (not Equal to)  (/= numstr [numstr …])  Returns T if the arguments are not numerically equal, and nil if the arguments are numerically equal 
> (greater than)  (< numstr [numstr …])  Returns T if each argument is numerically less than the argument to its right, and returns nil otherwise 
>= (greater than or equal to)  (<= numstr [numstr …])  Returns T if each argument is numerically less than or equal to the argument to its right, and returns nil otherwise 
< (less than)  (> numstr [numstr …])  Returns T if each argument is numerically greater than the argument to its right, and returns nil otherwise 
<= (less than or equal to)  (>= numstr [numstr …])  Returns T if each argument is numerically greater than or equal to the argument to its right, and returns nil otherwise 
and  (and [expr …])  Returns the logical AND of a list of expressions 
boole  (boole func int1 [int2 …])  Serves as a general bitwise Boolean function 
eq  (eq expr1 expr2)  Determines whether two expressions are identical 
equal  (equal expr1 expr2 [fuzz])  Determines whether two expressions are equal 
or  (or [expr …])  Returns the logical OR of a list of expressions 
angtos  (angtos angle [mode [precision]])  Converts an angular value in radians into a string 
ascii  (ascii string)  Returns the conversion of the first character of a string into its ASCII character code (an integer) 
atof  (atof string)  Returns the conversion of a string into a real 
atoi  (atoi string)  Returns the conversion of a string into an integer 
chr  (chr integer)  Returns the conversion of an integer representing an ASCII character code into a singlecharacter string 
itoa  (itoa int)  Returns the conversion of an integer into a string 
rtos  (rtos number [mode [precision]])  Converts a number into a string 
Lists and Loops contain the core LIST Processing commands
type  (type item)  Returns the type of a specified item 
eval  (eval expr)  Returns the result of evaluating an AutoLISP expression 
quote  (quote expr)  Returns an expression without evaluating it 
repeat  (repeat int [expr …])  Evaluates each expression a specified number of times, and returns the value of the last expression 
while  (while testexpr [expr …])  Evaluates a test expression, and if it is not nil, evaluates other expressions; repeats this process until the test expression evaluates to nil 
if  (if testexpr thenexpr [elseexpr])  Conditionally evaluates expressions 
cond  (cond [(test result …) …])  Serves as the primary conditional function for AutoLISP 
foreach  (foreach name lst [expr …])  Evaluates expressions for all members of a list 
progn  (progn [expr …])  Evaluates each expression sequentially, and returns the value of the last expression 
command  (command [arguments] …)  Executes an AutoCAD command 
defun  (defun sym ([arguments] [/variables …]) expr … )  Defines a function 
setq  (setq sym1 expr1 [sym2 expr2 …])  Sets the value of a symbol or symbols to associated expressions 
getvar  (getvar “varname”)  Retrieves the value of an AutoCAD system variable 
setvar  (setvar “varname” value)  Sets an AutoCAD system variable to a specified value 
load  (load filename [onfailure])  Evaluates the AutoLISP expressions in a file 
*error*  (*error* string)  A userdefinable errorhandling function 
list  (list [expr …])  Takes any number of expressions and combines them into one list 
length  (length lst)  Returns an integer indicating the number of elements in a list 
last  (last lst)  Returns the last element in a list 
nth  (nth n lst)  Returns the nth element of a list 
reverse  (reverse lst)  Returns a list with its elements reversed 
car  (car lst)  Returns the first element of a list 
cdr  (cdr lst)  Returns the specified list, except for the first element of the list 
caddr  
cadr  
cons  (cons newfirstelement lst)  The basic list constructor 
append  (append lst …)  Takes any number of lists and runs them together as one list 
member  (member expr lst)  Searches a list for an occurrence of an expression and returns the remainder of the list, starting with the first occurrence of the expression 
assoc  (assoc item alist)  Searches an association list for an element and returns that association list entry 
apply  (apply function lst)  Passes a list of arguments to a specified function 
mapcar  (mapcar function list1 … listn)  Returns a list of the result of executing a function with the individual elements of a list or lists supplied as arguments to the function 
lambda  (lambda arguments expr …)  Defines an anonymous function 
Strings and File I/O are pretty straighforward
read  (read [string])  Returns the first list or atom obtained from a string 
strcat  (strcat [string1 [string2 …])  Returns a string that is the concatenation of multiple strings 
strlen  (strlen [string …])  Returns an integer that is the number of characters in a string 
substr  (substr string start [length])  Returns a substring of a string 
vlstringsubst  (vlstringsubst newstr pattern string [startpos])  Substitutes one string for another, within a string 
open  (open filename mode)  Opens a file for access by the AutoLISP I/O functions 
readchar  (readchar [filedesc])  Returns the decimal ASCII code representing the character read from the keyboard input buffer or from an open file 
readline  (readline [filedesc])  Reads a string from the keyboard or from an open file 
writechar  (writechar num [filedesc])  Writes one character to the screen or to an open file 
writeline  (writeline string [filedesc])  Writes a string to the screen or to an open file 
close  (close filedesc)  Closes an open file

The last category is Autocad graphic interfacing commands. ( I am leaving out a few things I think may be less useful or obsolete or inadvertently)
angle  (angle pt1 pt2)  Returns an angle in radians of a line defined by two endpoints 
distance  (distance pt1 pt2)  Returns the 3D distance between two points 
inters  (inters pt1 pt2 pt3 pt4 [onseg])  Finds the intersection of two lines 
osnap  (osnap pt mode)  Returns a 3D point that is the result of applying an Object Snap mode to a specified point 
polar  (polar pt ang dist)  Returns the UCS 3D point at a specified angle and distance from a point 
getangle  (getangle [pt] [msg])  Pauses for user input of an angle, and returns that angle in radians 
getdist  (getdist [pt] [msg])  Pauses for user input of a distance 
getint  (getint [msg])  Pauses for user input of an integer, and returns that integer 
getorient  (getorient [pt] [msg])  Pauses for user input of an angle, and returns that angle in radians 
getpoint  (getpoint [pt] [msg])  Pauses for user input of a point, and returns that point 
getreal  (getreal [msg])  Pauses for user input of a real number, and returns that real number 
getstring  (getstring [cr] [msg])  Pauses for user input of a string, and returns that string 
entsel  (entsel [msg])  Prompts the user to select a single object (entity) by specifying a point 
entdel  (entdel ename)  Deletes objects (entities) or restores previously deleted objects 
entget  (entget ename [applist])  Retrieves an object’s definition data 
entlast  (entlast)  Returns the name of the last nondeleted main object in the drawing 
entmod  (entmod elist)  Modifies the definition data of an object 
entnext  (entnext [ename])  Returns the name of the next object in the drawing 
entupd  (entupd ename)  Updates the screen image of an object 
ssadd  (ssadd [ename [ss]])  Adds an object (entity) to a selection set, or creates a new selection set 
ssdel  (ssdel ename ss)  Deletes an object (entity) from a selection set 
ssget  (ssget [mode] [pt1 [pt2]] [ptlist] [filterlist])  Prompts the user to select objects (entities), and returns a selection set 
sslength  (sslength ss)  Returns an integer containing the number of objects (entities) in a selection set 
ssmemb  (ssmemb ename ss)  Tests whether an object (entity) is a member of a selection set 
ssname  (ssname ss index)  Returns the object (entity) name of the indexed element of a selection set 
grread  (grread [track] [allkeys [curtype]])  Reads values from any of the AutoCAD input devices 
grclear  
graphscr  (graphscr)  Displays the AutoCAD graphics screen 
grdraw  (grdraw from to color [highlight])  Draws a vector between two points, in the current viewport 
grtext  (grtext [box text [highlight]])  Writes text to the status line or to screen menu areas 
menucmd  (menucmd string)  Issues menu commands, or sets and retrieves menu item status 
prin1  (prin1 [expr [filedesc]])  Prints an expression to the command line or writes an expression to an open file 
princ  (princ [expr [filedesc]])  Prints an expression to the command line, or writes an expression to an open file 
(print [expr [filedesc]])  Prints an expression to the command line, or writes an expression to an open file  
prompt  (prompt msg)  Displays a string on your screen’s prompt area 
redraw  (redraw [ename [mode]])  Redraws the current viewport or a specified object (entity) in the current viewport 
terpri  (terpri)  Prints a newline to the Command line 