Dartmoor Letterboxes by Anne Swinscow
okay, the book is from 1984 and her husband is on the front in lederhosen but it's like the bible for letterboxers and it's like one of the only books ever written on letterboxing. i had even tried to buy this book online in the states and it was out of print. we couldn't believe our hostel had it lying around. all three of us read it while we were there.