There are an increasing number of books that will be of interest to the folly lover - all sorts of niche titles about gatehouses, animal houses, mazes, modern themed gardens, grottoes, and general curiosities, but, very few purely about follies, and even fewer that are actually in print. I'll attempt to list all that are known, and if their titles are underlined, there'll be a link to a supplier. Some that are officially out of print are still easily available from places like the Amazon marketplace, and many older titles crop up on ebay.
For UK residents, there is really only one book you really need:

FOLLIES, GROTTOES AND GARDEN BUILDINGS by Gwyn Headley and Wim Meulenkamp
Published 1999 by Aurum Press, London
ISBN 1-85410-625-2
608 pages, b &w photos throughout.

The almost definitive guide to follies throughout England, Scotland and Wales. Absolutely essential to any folly hunter, with loads of photos, but also very lively, humorous and opinionated. I say almost definitive because there are a few they've left out, just to make your days out more fun..