The Sierra Club's Grassroots Organizing Training Manual is the first book an environmental activist-in-training should read. It's very concise, but it covers all the basics of organizing a campaign.

A printed version was previously available free of charge from the Sierra Club, but it's out of print now. The good news is that a .pdf version is available online.

This is a similar to the Sierra Club manual described above, but it's longer and much more complete. Highly recommended.

Randy Shaw is a long-time organizer for the homeless and low-income tenants in San Francisco's Tenderloin district. He is also an attorney. This is not a complete, comprehensive book covering all the information an activist-organizer needs to know, but it contains a lot of great advice from an extremely experienced activist.