Hiya
I really find Belerion and Cornovia by Craig Weatherhill to be fantastic resources, as seen here:
http://www.geniusloci.co.uk/inspiration.htm
As well, of course, as the usual TMA and Aubrey Burl... :)
Another favourite of mine (and on that same link) is 'Fentynyow Kernow (In Search of Cornwall's Holy Wells)' by Cheryl Straffon - rather uneliable regarding directions to some of the wells, but a great inspiration. And while we're on the subject (it doesn't take me much!) 'Secret Shrines' by Paul Broadhurst - also on that page - is a beautiful book to own...
G x