Burl's From Carnac to Callanish - The Prehistoric stone rows and avenues of Britain Ireland and Brittany, should not be forgotten.

I have Thoms Megalithic sites in Britain and his Megalithic Lunar Observatories. The text is pretty un-penetratable to my none mathmatical brain but the plans of circles can't be beaten.

I would strongly recommend Bob Trubshaws Sacred Places - Prehistory and Popular Imagination. Don't let the cover put you off, it's the best book I've read for yonks and would go so far as to say it is pretty damn close to essential reading for anyone who posts on TMA.

I'm a big fan of Richard Bradley and would definitely recommend any of his books particularly The Significance of Monuments.