90's rap instrumentals. Specifically most of J Dilla's unreleased discography, the MPC 3000 track 34 comes to mind, although a lot of his released stuff is great too. Favourite of mine is Dreamy, and the non-instrumental version is a remix of a D'Angelo song that I like, so it all adds up.
Nujabes is an obvious choice, LJones has a few nice tracks, and 9th Wonder was basically inspired by that entire era so his stuff captures it perfectly. There's a bunch of homework mixes on Youtube that stretch for an hour, they're probably the best study music I've come across (For me, that is) and they're kinda the reason I make instrumental techno stuff now.
These guys are all repetitive and unobtrusive in their production, which is exactly what you want for study music.