"Let's Go Mexico" has been running with the big dogs (Rough Guide & Lonely Planet) for years and it is the best guide out for those on a "starving student" budget.
There is a great section at the beginning of the book, "Discover Mexico", which has five suggested itineraries - excellent. The "Beyond Tourism" chapter will help those looking for volunteer opportunities, environmental preservation, social action, teaching English and studying everything from language to college level courses. These two sections alone are worth the price of the book.
The authors will find you the best rock bottom, dirt cheap, bargain basement, lowest price possible accommodations and restaurants to match. The accommodations listed in this guide span from under $10 to $25 plus. Where the guide has the "thumbs-up" symbol near the listing it is worth checking it out.
There are restaurants in the guide for $2 (this doesn't include your beer) and these eateries have very good food! The guide was written by six "research-writers" so one geographical area can be better described than another. I found the descriptions of both the accommodations and eateries to be very brief and not as informative as the upscale guide ,"Frommer's" (a very good guide for Mexico - strongly recommended). However, you still get enough information to make a discerning choice.
For those who love "Nightlife" (bars & clubs), the "Let's Go Mexico" guide is the best out today... especially for the young crowd. All of their recommendations, including the bars and clubs, are easy to find on fairly decent maps.
Bottom line: This is guide is especially valuable if you are trying to explore Mexico on your lunch money. Happy Trails. Highly Recommended.