You might look for (shameless plug coming) my new Guide to Mainland Belize. It covers just about every part of the mainland, in 288 pages with 75 photos and maps. Available from Amazon.com.
Ambergris Caye and the other cayes are just the tip of the "iceberg" as it were in Belize -- the mainland is the real Belize. There's really too much to see in a month, much less a day or two. And of course it all depends on what your interests are -- birding or wildlife spotting, Maya ruins, caving, hiking, cultural tourism or whatever. There are more than a dozen major Maya sites (and hundreds of minor ones), scores of waterfalls, six butterfly farms, a dozen national parks, many interesting villages and towns, just as a start.
Belize First Magazine http://www.turq.com/belizefirst/