I'm a bit late on this, and can recommend what others have said here.

Romance of the Stones is the most comprehensive, but you may have problems finding a copy these days. £45 for the hardback, £20 for the paperback and worth every penny!

Men-an-Tol Studio guides are v. useful for Penwith too, as are Cheryl Straffon's books. You can ask to speak to her in St Just Library most days - I think she keeps a stock there too.

Craig Wetherill's Belerion and Cornovia provide quite comprehensive guides and can be indispensible.

One book that hasn't been mentioned yet is Ian McNeil Cooke's Mother and Sun: The Cornish Fogou. Although a limited edition, it's still possible to find signed copies of this around. The Cornish Shop in St Just and the museum in Truro both had copies a couple of weeks ago.