Within Emacs, you can type "C-h f" to get the documentation for a function, "C-h v" for a variable.
For more information, obtain the GNU Emacs Lisp Reference Manual for Emacs 18 under Unix. It is available from the FSF for $50 (or 5 for $200). The latest revision available for FTP is edition 1.03 dated 28 January 1991.
For online use, a set of pregenerated Info files is available with the Texinfo source for the Emacs Lisp manual via anonymous FTP:
(You can also create the Info files from the Texinfo source.) See section How do I install a piece of Texinfo documentation? for details on how to install these files online.
If you are daring enough to try to print this 550 page manual out yourself, for instructions see section How do I print a Texinfo file?.
Also, as a popular USENET saying goes, "Use the Force, Read the Source".