Introduction
Belize is a part of the "Ruta Maya". A couple of years ago, it was the only route to go from Flores to Mexico, but nowadays you can also go to Palenque instead.
We went through Belize to Mexico, so here's a report on the 2 places we stopped: San Ignacio just over the border, and Caye Caulker a nice little island to relax. Belize was in my opinion not as nice as Guatemala. Everything is more expensive. A lot of the resorts there are catering for rich Americans who come there to spend their 1 week of holiday, but low budget accomodation can be found everywhere.
Getting there from Flores
Best thing is to take the public bus from Flores to Melchor de Mencos, the border post for 10 Q (8/97). It leaves at 7.30 am, goes around town for 30 minutes and then finally leaves. The trip takes 2 hours. There are other buses as well.
Passing the border is pretty straightforward. Leaving Guatemala here costs 10 Q. You need to fill in a customs declaration. You can change money at the border, the rate is 2 B$ for 1 US$. TC are accepted. If it's a saturday or sunday, you should change here, since banks are closed.
A more expensive but also faster option is to take a minibus all the way to Belize city, or even Chetumal. This cost 20 US$. They connect with the speedboats to Caye Caulker.
Travel guides
If you are doing the typical Ruta Maya (Guatemala -Belize -Yucatan /Chiapas), then this book will give you good info in 1 book. However, it was clear to us that the LP guide came on the 3rd place. The prices where outdated, but this is inevitable. Also the info was clearly not as detailed as the Rough guide f.i (this review was based on the '98 book). New edition since 3/2001).
Order the Lonely Planet Belize, Guatemala, Yucatan now for only 15.96 US$!
Or maybe you prefer their Central America Book
Order the Lonely Planet Central America on a shoestring for 15.96 US$!
LP has an excellent guidebook on diving and snorkeling in Belize
Order the Lonely Planet Diving & snorkeling in Belize for 12.76 US$! (edition 9/98)
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Footprint Handbook Central America-Mexico
This book had a lot of practical info and was definitely the best on accommodation and restaurant info. Their info seemed also more up to date (there's a new edition each year in december). I recommend it especially if you need middle class accomodation.

Order the Footprint Central American Handbook for 19.57 US$!
Rough Guide to Belize
Edition March 1999 At the time I read the book, it was together with the Guatemala section, but they're seperate now. This was my personnal favourite with very detailed info. Simply compare the number of pages here with the LP guide and you'll understand it has much more info.

Order the Rough Guide Belize now for only 13.56 US$!
The Rough Guide series now have a competitor of LP's Ruta Maya (June 1999), called "Maya World". They have here info on Guatemala, Belize, and Yucatan, but also El Salvador and Honduras. I read parts of this book and it's excellent value for money, better than LP in my point of view.

Order the Rough Guide Maya World now for only 15.96 US$!
Other books

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Wegwijzer
For those of you speaking dutch, it is definitely worth considering becoming member of "Wegwijzer". This is a non-commercial organisation based in Bruges, Belgium. They have an incredible amount of info coming from guidebooks, leaflets but also reports from members. Since 2001 you can now get a lot of these reports online!! Membership is only 25 Euro a year, and this includes a good magazine they publish every 2 months. Have a look at http://www.wegwijzer.be/default.htm. I myself am a member since almost 20 years now.
Go to San Ignacio