And don't forget that Cornovia has a companion volume: Belerion : ancient sites of Land's End - ISBN: 090672001x
I found the two invaluable on my recent trip down there. And if you can afford it, there's The Romance of the Stones (Cornwall's Pagan Past) by Robin Payne - ISBN: 1899526668 which catalogs *all* (!) of the extant stones, and mentions quite a few more that are no more...
Hmm, maybe we need a book recommendation section on TMA...