Courtesy of Marc Puckett, I read at the New Liturgical Movement about Usuarium, an online database which catalogues over 800 liturgical books from the last thousand years or so which are available to download online and, more importantly are tagged and searchable by Use, by liturgical ceremony, by country of origin. It is, simply, as complete a resource as anyone interested in liturgical history could possible need.
What were the prayers at the foot of the altar like in the Use of St Andrews? How similar is a Roman Missal of the early sixteenth century to the post-Trent version? Get an account, log in and all will be revealed.