In terms of paper maps, there is the Ordnance Survey map (1:50,000-scale) done in the early 90s. We sell it (for US$14) but candidly I don't think it's that useful, and in any case it is badly out of date.
About the best map to the island is Savanna's Bohemian Map, a hand-drawn map with advertising, but it was done in 1995 and I don't think it's ever been updated. You can buy it on the island at some places for five or six bucks.
Besides the electronic maps on Marty's site, I have the maps from my Free, Fun & Easy Guide to Ambergris Caye on our Web edition of Belize First -- got to http://www.turq.com/belizefirst/amcay/amberg.html
for the Ambergris Caye guide and maps.
Belize First Magazine http://www.turq.com/belizefirst/