The English Folk Dance and Song Society and the Folklore Society have recently published Room, Room Ladies and Gentlemen: an Introduction to the English Mummers' Play (ISBN 9 780854 181858), written by Eddie Cass and Steve Roud. I'd recommend it as a very useful and clearly written introduction; it's aimed largely at teachers and schools but is also helpful for any newcomer to the subject. Current research seems to point to an origin of the custom somewhere in the early 18th or late 17th centuries (certainly not "the Middle Ages"), but there is more work to be done of course. The Victorian preoccupation with the supposed pagan origins of traditional customs is largely discredited now, though still clung to by many romantics.