[Anaraxes]

Or rather than base everything on a single core skill like Thaumatology or Ritual Magic, go the "Skills For Everyone" route, and require a skill per spell to use it, much as was done for Psionic Powers. Define those skills as something that requires book learning to improve, not just repeated casting of the spell. Some such individual spell-skills might well have some that can be learned only from lost or ancient books, as they're unknown to the academic wizard community.