Also see the links under Glossary
http://www.bl.uk/catalogues/istc/index.html - British Library Incunabula Short Title Catalogue. A searchable database of 15th-century European printing that records nearly every item printed from moveable type before 1501.
http://www.lib.latrobe.edu.au/MMDB/index.htm - La Trobe University medieval Music Database. A searchable database of liturgical chant from the 10th to the 15th century.
http://www.anzaab.com - Australian & New Zealand Association of Antiquarian Booksellers.