Real history

While the setting of Ars Magica is not historical Europe, Mythic Europe is heavily influenced by our real world's history and myths. Fortunately, there is no shortage of sources of information on these matters. Educating yourself and playing with history and folklore is part of the pleasure of playing Ars Magica.

General Sources

The following cover the history of large parts of Europe and are relevant to most Ars Magica sagas.

The Catholic Encyclopedia: The Catholic bias is just the right one for an Ars Magica game. The encyclopedia is a great introduction to the many heresies, the structure of the Church, theological issues, and so on.
Thomas Aquinas' Summa Theologiae: The main source for Thomist scholastic thought, translated into English. Written as a textbook for medieval students, hence remarkably accessible. The place to find explanations of scholastic, ecclesiastical, theological or philosophical concepts like species or heresy.
Entry on Aristotle in the Stanford Encyclopedia of Philosophy: Aristotle - 'the philosopher' - is the authority scholastic thinkers quote most. This article gives a thorough overview of his work and its tradition.
Folklore and Mythology Electronic Texts: Thanks to D. L. Ashliman and the University of Pittsburgh, a great wealth of folk tales is available at your fingerprints. Following his links will lead you to other interesting places.
The Medieval Bestiary: Illustrated online bestiary combined from two major medieval bestiaries. You'll find the source for many Beasts of Virtue as well as ideas for monsters, familiars, Animal Companions, and more.

Maps

Historical Maps of Europe: Historical maps of Europe from the University of Texas at Austin

Regional Sources

Ben Cooper's German Timeline: Timeline of events affecting the Rhine Tribunal, 1220-1231.

Offline Biblography

These sources are unfortunately not available online. However, a visit to your local library or a few purchases can bring you in touch with them.