My good friend (and mentor years ago) Richie Woods lives in San Pedro and he did a field guide on the wildlife, archaeology, shipwrecks, etc on Ambergris. It still serves as a pretty good referenc. I will try and find the link to it if it still exists on the web.
Also, as part of the process of establishing Bacalar Chico as a marine reserve, a biological survey was done of the terrestrial habitats and there is a report on that too. I think it was done by Simon Zisman and Coral Cay Conservation (they home office is in London) and you may be able to get a copy there. I may have a copy, but it would be in Belize.