The Kilmartin House Trust published a big soft-backed glossy book called "Kilmartin - Scotland's Richest Prehistoric landscape - An Introduction and Guide " by Rachel Butters with photographs by David Lyons.
At about 110 pages it is NOT an exhaustive survey or a particularly detailed exploration of the area. It is very well illustrated and covers the Kilmartin Valley's main sites and looks at it in the wider context of Argyll.

It is here for the princely sum of £8.90 with free delivery.

http://www.amazon.co.uk/Kilmartin-Scotlands-Prehistoric-Landscape-Introduction/dp/095336741X

It is worth buying for the Foreword by Marion Campbell of Kilberry who wrote up a seminal study on Kilmartin - "Mid Argyll : A Field Survey of the Historic and Prehistoric Monuments" back in 1963. In the foreword she explains how her original 1963 study listed and covered 640 sites, a catalogue of finds, four distribution maps and a graph of heights. Sadly Marion is no longer with us but she tells us her original field report was "the product of two untrained women's scanty leisure with a notebook, a six foot steel tape and a 33 foot suveyor's tape".
Another book you might want to have a wee look at is "Stone Voices" by Neal Ascherson. He writes about Kilmartin (and many other things scottish) and he writes about Marion Campbell's last days in the hospital at Oban with humour and heart. He describes Marion as "an historian, novelist and poet; she was a patriot antiquary, a sailor in war, and a farmer in peace; she was the mother of scientific archaeology and of community museums in Mid-Argyll"
Marion Campbell also solved a mystery regarding the fallen Ballymeanoch stone. "It was in 1943 and a Shetland Pony was sheltering up against it from a storm when it broke off. Must have terrified the poor beast! Nobody would believe me that I remembered that stone when it was up and how I used to look through the hole."