If I might, I would recommend the following reading to the OP:
Warder, A.K., Indian Buddhism, Motilal Banarsidass Publishers, 3rd edition, 2000 (2008 for reprint).
Great reading, quite accessible, and it covers "original" Buddhism presupposed by the traditions of the different schools known to us, and the development of the latter ones and how they modified the original doctrine. A.K. Warder is a great scholar on that topic.
Sorry about my english, not my mother tongue.