The Foundations of Buddhism, Oxford University Press, Rupert Gethin
This is an excellent, even handed treatise on the history of Buddhism, and the differences between the different lineages. It is a little academic, but nonetheless it is highly recommended.
Buddhism, an Illustrated Guide, Oxford University Press, Kevin Trainor
This is also an excellent, general text on the history of Buddhism, as well as a survey of the Buddhist landscape. Note that Professor Trainor is in the school of religion at the University of Vermont.