This book is invaluable on any holiday or trip to the countryside (or even to town), because it makes you realise there are follies _everywhere_ in Britain. Look up the area you're going and you'll find, in a hidden side road, the most remarkable and unexpected building you'd otherwise have missed. We drove past Wattisham Castle, an obscure farm in Suffolk disguised with towers and traceries to look like a Disney castle: the newish owners (doing it up) were astonished to find their house in a book and treated us to tea.
This book isn't a dry encyclopedia: as well as being highly informative, it's peppered with a deep, subtle wit which makes it a very entertaining read. (A random sample, on Lord Ducie's house at Tortworth, Avon: "When the house because Leyhill Open Prison the word 'WELCOME' over the archway was removed". One of my favourites on p.195-196 is their description of two follies built - clearly at some expense - to display two criminally dreadful poetic inscriptions. I won't spoil it, you just have to read it.)
For a walk in the country, just take this book and the Good Pub Guide.