I think the two best Belize guidebooks currently in stores are The Rough Guide to Belize by Peter Eltringham and Belize Handbook from Moon Publications, by Chicki Mallan and her daughter. Both are reasonably current although the authors are both in Belize now updating the guides, which will have new editions late this year. Both Peter and Chicki are subscribers to my magazine, and Peter is a friend. Peter has lived in Guatemala and Belize (he first came to Belize with the British Army) and knows an incredible amount about the country.
I recently updated for Random House Fodor's Belize & Guatemala Guide and also UpClose Central America. Those will be out later this year and, with the limitations that they cover areas other than Belize are both really well-researched guides. I added a great deal about Ambergris Caye to both of these guides.
Specifically on Ambergris Caye, my Fun, Free & Easy Guide to Ambergris Caye is on our Web edition of Belize First Magazine. I need to update it a little, but it's current as of about mid-1999, probably more current than any of the guidebooks now out, which were mostly researched in 1998 for publication in '99.
There are several books which cover Ambergris Caye history and non-travel information, including the Field Guide to Ambergris Caye. These are available on the island.
Belize First Magazine http://www.turq.com/belizefirst/