We used a number of guide books purchased before we left the USA. I made photocopies of the pages we needed, and we took a manila envelope filled with the photocopies on our trip. That way we didn't have to haul around the weight of the books. On days that we visited a site, I would take the pages for that site, fold them up, and put them in my pocket. It was a lot lighter than carrying books around. We used four different guides, but if I had to recommend only one, it would be the "Lonely Planet guide to Guatemala, Belize, & Yucatan". Make sure you get the Third edition, which came out in 1997 (after my trip). We unfortunately had the 1994 edition, but it still had the best info and tips.