The Manual of Tourist Entry was put together by Mexico's Secretariat of Tourism in conjunction with the government agencies that a tourist will come into contact with whether entering by land, sea or air. Some of the topics covered are migration, customs and prohibited products, importing vehicles and airplanes, and a list of government phone numbers including each state's tourism office. The manual is available in English and Spanish on the Secretariat of Tourism (SECTUR) website.
The SECTUR website is also a good resource for information on Mexico's highways and the Green Angels.