What do you want in a travel guide? I want plenty of information about the sights and events so that I can plan my trip before I go. Great photos help with this stage, and I like to be able to get a sense of prices at this point, too.
On the way to my destination, I like to have lots to read: information about the history of the place I'm visiting, for example, and details of the customs and characteristics of the region. Knowing these things enriches the experience for me, and helps beguile the time in airports, too.
Once I've arrived, I want maps. Lots of maps. I don't want to have to rely on having access to Google Maps on my phone, and I like to be able to write on the maps. Useful phrases in the local language are helpful, too, and a heads-up on any laws I might unwittingly break or dangers I might not consider are also good.
Lonely Planet's Puerto Rico includes all these things. It's a satisfying read for the armchair traveler, covering music, dance, politics, sports, cuisine, and daily life in Puerto Rico. It's also a terrific resource when you need a swift answer on dengue fever or San Juan's laws about drinking in public.
Above all, it's a guidebook. Each region of the island is presented with large-scale maps, street maps, and lists of sights, restaurants, accommodations, and shopping. Opportunities for outdoor sports and nightlife are given, and there are lots of special tips and bits of interesting information, from where to watch out for mosquitoes to the best places to look for ATMs.
The book is well-written, small enough to tuck in your backpack, and contains a full-size pull-out map.