There are a small number of widely available and relatively similar (in size and content) glossy travel guides for Belize - the top five most recent publications (as of Jan 2014) are below.  Everyone has their favourites! 

Lonely Planet Belize by Mara Vorhees, Joshua Samuel Brown (2013) 

Belize - Moon Handbook by Lebawit Lily Girma (2014)

Frommer's Belize by Eliot Greenspan (2011)

The Rough Guide to Belize by Peter Eltringham (2010)

Insight Guide Belize by Brian Bell (2008)

Keep a lookout for the excellent plasticated loose leaf Belize Field Guides on sports fish, reef fish, mammals, reptiles and birds. 

Trees of Belize by Kate Harris (2013, second edition).  For natural history enthusiasts, tourists and locals alike; a comprehensive and concise field guide detailing seventy of Belize's most significant species from allspice to ziricote - thoughtfully written and beautifully illustrated. It provides an enjoyable insight into the stunning diversity and splendour of Belize's tropical flora.  Available in-country and amazon.uk

Also check out One Hundred Healing Herbs Of Belize by Rosita Arvigo and Michael Balick (1993) or Rainforest Home Remedies: The Maya Way to Heal Your Body and Replenish Your Soul by Rosita Arvigo and Nadine Epstein (2001) available in-country. 

Belize Islands Guide: Guide to Ambergris Caye, Caye Caulker and the Offshore Cayes and Atolls of Belize by Lan Sluder (2010)

Books to read in your leisure time...

The Last Flight of the Scarlet Macaw by Bruce Barcott (2008).  Tells the story of Sharon Matola's fight to stop the Chalillo dam on the Macal River.  Matola is the founder and director of the Belize Zoo, and the dam destroyed important habitat of the fragile and endangered Belizean macaw population.

For archaological fans; the Chronicle of the Mayan Kings and Queens: Deciphering the Dynasties of the Ancient May by Simon Martin and Nikolai Grube (2008).  It has more indepth detail on several of Belize's Mayan sites.