I think fantasy in general made me interested in Arthuriana, folklore and mythology, and medieval studies (which was my college major), but I can't think of a specific book that triggered those interests. Maybe the Deryni books tilted me more into medieval studies than anything, though?
I've had several to be honest. The one that leaps out at me is His Majesty's Dragon by Nomi Novak.
I think fantasy in general made me interested in Arthuriana, folklore and mythology, and medieval studies (which was my college major), but I can't think of a specific book that triggered those interests. Maybe the Deryni books tilted me more into medieval studies than anything, though?