oh so many... my first introduction to RA was using the Stan Beckensall* books - firstly "Prehistoric Rock Art in Northumberland" then "British Prehistoric Rock Art". Depends what you are after ... the British one will get you a nice general intro

list of his books here :
http://en.wikipedia.org/wiki/Stan_Beckensall

NB - Rock art is a fast-moving, organically-growing pacey subject - those books just scratch the surface and were already out of date as soon as they were published!!

*other Rock Art authors are available and will surely speak up ;)