These four books are absolutely essential for touring Mexico as we did. First is the Church's "Traveler's Guide To Mexican Camping" Every Mexican RV'er has one and every one is as dog eared as mine. It describes and gives the location and price of every RV Park in Mexico and is very accurate. Amazon .ca (Amazon.com) has them. On the road it is known as "The Bible", Don't leave home without it.
Next is the Guia Roji Road Map Book. You can find this at any OXXO Store in Mexico. Many Pemex Gas Stations have an OXXO alongside. They cost $190 Pesos or about $18 CAN. It is "THE" Mexican road map book!
This is the Lonely Planet Guide to Mexico. You can order it from Amazon.com (or Amazon.ca) before you leave. We also bought the Fromers guide but it only covers the large cities. Lonely Planet Guide has all cities and villages. Their hotel recommendations are spot-on and the prices quoted are accurate.
Have an English / Spanish dictionary with you all the time. You will find that in the interior of Mexico very few people speak any English at all. You will need to carry the book with you so don't buy one that will not fit in your pocket. If you are trying to read a menu the book will do you no good back in the motorhome or hotel! This one is put out by Lonely Planet and is good, but there are many others. Just make sure it fits in your back pocket. Maybe even have two for when you loose one.