Written by a Catholic, its fervent belief may bother some readers, but if you want to know how medieval liturgies and such went, this is excellent. He covers them as they develop from the apostolic days (as the Church of Rome saw it). His ideas about how knights fought are kind of cockeyed (that they only rode their destriers to the battlefield then fought dismounted, something no serious art historian should suggest, and never have I seen it elsewhere). On the whole, a great companion piece to his Manners, Customs, and Dress of the Middle Ages and the Renaissance, filling in the holes there.

PS: his name is not Jacob, P. L., but Lacroix, Paul. The initial listing here is so wrong.