On the Web
"Eric Weisstein's World of Mathematics"
This Web site is like a giant comprehensive math book you can find information on just about any math topic here.
CRC Standard Mathematical Tables and Formulae, 30th Edition
By Daniel Zwillinger
ISBN: 0849324793; $51.95; www.crcpress.com
A must-have for anyone who uses math frequently. It does not teach any math; it's just the most comprehensive math reference I have ever seen. It is a required book for many hard-science degree programs.
Schaum's Outline of Trigonometry
By Robert E. Moyer and Frank Ayres, Jr.
ISBN: 0070068933; $15.95; www.mcgrawhill.com
The Schaum's Outline books are like Cliffs Notes for math and science. They are concise references in an easy-to-use format.