Aeolius said:Flumphonomicon

He's got something pretty close to that already, called Superstitions.Lord Tirian said:Mysterica Medieval
A comprehensive guide to the supernatural middle-ages.
Detailing a host of superstitions, beliefs, legends, and myths from our own past.
Full of information to make a supernatural middle-age believable, yet recognizable.
Includes new d20 classes and spells to model the archetypical priests and hermetic mages.
Mark CMG said:I think my next project is going to rock huge for gaming in 3.5 games, as well as any that utilize a medieval/fantasy campaign setting.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.