Heresy and Inquisition
Catholic Encyclopedia: Inquisition
Extensive article by Joseph Blötzer provides a general definition and history of the Inquisition and attempts to explain its evolution and methodology within Church and secular law.
Heretics, Heresies and the Church
A general introduction to the trends of religious thought by Steven Kreis.
Extremely helpful overview by David Burr defines the medieval view of the Inquisition and covers its establishment, aims, methods, and more at Paul Halsall's Medieval Sourcebook.
Mystics and Heretics
An overview of the progression of heresies throughout medieval Christianity, by Professor Gerhard Rempel, Western New England College.