Though not really that much connected to megaliths I have really enjoyed these two books by Ronald Turnbull. I like looking at the different rocks/stones that are used in various stone circles, chambered cairns, etc but my knowledge is pretty poor as to what each type actual is. I find some geology books a bit hard work but these two are quite easy going with some lovely photos. One covers coastal geology (mainly south of the border) and the another the mountains.

Sandstone and Sea Stacks: A Beachcomber's Guide to Britain's Coastal Geology
http://www.amazon.co.uk/Sandstone-Sea-Stacks-Beachcombers-Britains/dp/0711232288/ref=la_B002IYV75S_1_17?s=books&ie=UTF8&qid=1370871021&sr=1-17

Granite and Grit: A Walker's Guide to the Geology of British Mountains
http://www.amazon.co.uk/Granite-Grit-Walkers-Geology-Mountains/dp/071123180X/ref=la_B002IYV75S_1_4?ie=UTF8&qid=1370870508&sr=1-4