Mostly ancient legends and magic and stuff. He once wrote a huge thesis while in prison (long story) about Hero Relics (artifacts of ancient heroes that hold great power), their legends, where they'd be found, and how they can be used to bring about world peace.

In Johto, he's been focusing mostly on local tales about Legendary Pokémon, especially since meeting Raikou. He's also been looking a little at different Pokémon moves and how they work - like, how Thunderbolt is actually called forth, where the water for Water Pulse comes from, stuff like that - but it's not really anything serious. (...This is also partially living vicariously through his Clefairy, even if he'd never ever think of it that way and she's even more enthusiastic about learning new moves than he is about teaching her, but.)