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.