The most thorough and up-to-date guidebook that I’ve seen is The Bradt Guide to The Gambia by Craig Emms, Linda Barnett & Richard Human (June 2006). It is a comprehensive, insider’s job, with excellent maps.
Need a local guide? Freelance guide Junkung “JJ†Jadama can arrange custom tours of any duration for groups or individuals. He specializes in bird watching, nature, and cultural tours: www.gambiabirdguide.com.
Anyone have any other Gambia resources? Please post them in the comments.