Excerpts from Einhard's Life of Charlemagne.
http://www.fordham.edu/halsall/source/einhard1.html
With a link to the full text. The most reliable medieval source on Charlemagne's life and times.
The Online Medieval and Classical Library (OMACL).
http://sunsite.berkeley.edu/OMACL/
A collection of some of the most important medieval and classical texts. A good resource.