This question also is not appropriate to this newsgroup, but is frequently
asked. See the newsgroup misc.books.technical instead.  Here is a summary 
of resources:

A Concise Guide to UNIX Books
Compiled by: Samuel Ko (kko@sfu.ca, sko@wimsey.bc.ca)

This is a good selection of the "best" books and documentation on
UNIX and related areas.  The selection is based on i) recommendations
from netnews readers, ii) the US/Canada sales figures, and iii) a bit
of my own preference.

Among the subject areas covered:
   A. General Unix Texts
      a.  for beginning / intermediate users
      b.  for intermediate / advanced users
   B. Shells
   C. Unix Editors
   L. Other Lists
   M. Other Books (experimental)

The latest version is also obtainable by anonymous ftp from rtfm.mit.edu
(in /usenet/news.answers/books). If you do not have ftp or netnews
access you can get it by email from mail-server@rtfm.mit.edu and the body
of your request should be send usenet/news.answers/books/unix.