The Dancing Chain (History and Development of the Derailleur Bicycle) by Frank J Berto
Fantastic book. It concentrates mainly on the history of derailleurs, but it does go into some detail on different kinds of hub gears, and other exotic gearing systems like "retro direct" (pedal forwards for one gear, backwards for another) expanding chainwheels and so on.
PM me if you want a cut price copy, because due to an ordering blunder, I have two.