Someone may have already given you a link to this book but if not: http://www.amazon.co.uk/Kilmartin-Scotlands-Prehistoric-Landscape-Introduction/dp/095336741X/ref=sr_1_1?ie=UTF8&qid=1307363898&sr=8-1
I bought it from Kilmartin Museum last year and it's the best one on offer in terms of offering great content and illustrations.
Stunning place. Hoping to work on some paintings of this whole area soon.