A lot of new Travel books for Belize were printed in 2004/2005. As the tourism industry in Belize has completely changed over the past 3 years, the new books are very informative and will help you plan a really great vacation.

Belize - Moon Handbook by Joshua Berman (7th edition, Paperback, September 2007) 

Lonely Planet Belize (Lonely Planet Belize) by John Noble and Susan Forsyth ( Paperback - Nov 2, 2005)

The Rough Guide to Belize, Third Edition by Peter Eltringham ( Paperback - Nov 29, 2004)

This next book came out in 2003 but is excellent because it is not a book for planning your hotels etc. It is a reference book, all color, all glossy. It tells you about the history, Mayan Ruins, culture etc.

Insight Guide Belize (Insight Guides) by Huw Hennessy ( Paperback - Jul 2003)

Other suggestions:
For books to read in your leisure time...

Read Myths of Ethnicity and Nation Immigration, Work, and Identity in the Belize Banana Industry by Mark Moberg (ISBN 0-87049-970-X).  It is an interesting book, it's not for the casual reader before vacation as it's more depressing than uplifting in many cases.

While in Belize, another recommended reading is the Chronicles of the Mayan Kings and Queens.  It has several of the more in-depth tourist places and visitor centers.