Travel guides
The Moon Belize guide (various editions) is the most thorough printed guide to the country and is updated regularly. Lonely Planet covered Belize as part of its Guatemala, Belize & Yucatán guide for many years.
Culture and history
Belize: A Concise History by Brian Dyde gives a solid overview of the country's colonial past and road to independence.
Sastun: My Apprenticeship with a Maya Healer by Rosita Arvigo (ISBN 006250259X) is a memorable account of learning traditional Maya medicine from a healer in the Cayo district. Arvigo went on to found the Ix Chel Tropical Research Foundation and helped preserve vast tracts of jungle.
The Mopan: Culture and Ethnicity in a Changing Belizean Community by James R. Gregory (ISBN 0913134740) is an academic study of the Mopan Maya, useful background if you're spending time in the west.
Natural history
A Field Guide to the Birds of Mexico and Northern Central America by Steve Howell and Sophie Webb is the best birding reference for Belize. The country has over 600 recorded species.
Reef Fish Identification: Tropical Pacific — if you're diving the reef, a waterproof identification guide is worth packing.