I picked up Transit Maps of the World yesterday and have been enjoying browsing through it. Basically, it's a book of transit maps from all around the world! The book goes city-by-city and covers both large and small systems and mixes maps with text which describes both the history of the system and the history of efforts to graphically depict the system.
Normally people will probably find this book extremely dull and weird, but it makes an ideal gift for the transit enthusiast in your life. Obviously, buying copies of Heads in the Sand for everyone you know should be a higher priority, but the right kind of person (i.e., me) will love Transit Maps.