Jacqueline Simpson and Steve Roud
A Dictionary of English Folklore
Oxford University Press 2001 (2000)
For me the best reference books are those which not only provide a entry matching your initial query but which also encourage you to browse and read other not always related entries. This Oxford Dictionary does it for me on both counts: authoritativeness and readability. Folklore here is rightly interpreted as including aspects of modern popular culture as well as topics beloved of antiquarians.
Authored by two stalwarts of the Folklore Society — who should then know what they are talking about — the Dictionary contains over 1250 entries covering a wide range of topics including seasonal customs, traditional tales, superstitions and beliefs. Key figures involved in the recording of lore are noted here, and evidence presented that folklore is part of a continually evolving process. What makes this book particularly worthwhile is that not all so-called traditional lore is accorded uncritical acceptance, a plus for any truth-seeker when Victorian speculation about origins and meaning often became spurious fact.
For those wanting more there are relevant references and a bibliography, and in common with many in this Oxford reference series, pretty pictures are excluded in favour of more text. Sometimes this is a disadvantage but in this case I’d rather have more entries than a limited number of select and maybe unrepresentative illustrations. (Having said which, I include a curious photo of 19th-century mummers acting out their seasonal play.)