Penguin Guide to the Superstitions of Britain and Ireland by Steve Roud
This survey not only explains what people have believed and why, but when superstitions arose, which parts of the country adopted them, how they evolved and what people today believe. Drawing extensively on literary sources from medieval times to the present, the book settles many arguments, debunks many myths and provides in the process an interesting sideways view of social customs and beliefs over the centuries.