First off, take a look at our FAQ section for a discussion of books
I'm new to Unix. Which books should I read? - The UNIX and Linux Forums
Then, scan through some books at your local bookstore. Find one that matches your style of learning. For instance, do you just need a short handbook that contains commands and syntax, do you need a book to teach you how to solve problems (teach you computer logic), or do you need a book that mixes commands with examples.
In addition, I would invest in a SED/AWK book. That will round out you unix capabilites as those two can solve many problems for you.
On my shelf:
Unix Shell Programming, 3rd edition, by Stephen Kochan and Patrick Wood (publisher SAMS)
sed & awk, 2nd edition, by Dale Dougherty and Arnold Robbins (publisher O'Reilly)
vi Editor pocket reference, by Arnold Robbins (publisher O'Reilly)
Perl pocket reference, by Johan Vromans (publisher O'Reilly)