Cavalry Operations in the Ancient Greek World by Robert E. Gaebel
This is a thorough and competent overview of the subject working chronologically from the earliest real evidence of Greek cavalry to the Successors of Alexander the Great, with a chapter on Hannibal for good measure.
My main criticism is that I found the analysis over-dependant on Victor Hanson's unconvincing portrait of the Ancient Greeks as an exceptional culture and Phalanx warfare as an exceptional (and somehow egalitarian) activity, but this was not presented to the same extent as in say Hanson's own book in the Cassell History of Warfare series, and doesn't therefore make the book unuseable if you don't happen to share this view.
On the positive side is the author's very balanced view of the importance of cavalry, which draws out both the effectiveness of cavalry in the earliest era and their continuing limitations even under Alexander and the Successors. He is interested in more than just charging on the battlefield and demonstrates a healthy scepticism about the technological constraints (such as lack of stirrups) which are sometimes held to have impeded cavalry in this period.
I don't think its changed my life, but this is a book well worth buying.