I used the Rough Guide to Puerto Rico as a reference for my general knowledge about the island during my circumnavigation journey. The Rough Guides series are well written, provide useful insights about local culture, and points of interest you would otherwise miss.
This book is a useful companion to have with you when planning your paddle for the next day or two. If you are out of reception and stuck on a weather delay, it's also a good way to pass the time.
Before we all had internet on our phones, this book would have been the best source of information to have with you, other than your nautical chart.