If one asks him about his seriousness (bookwise):

"Of course I do. If I didn't, most of the books would've been burned to ashes or dissolved to nothing before the Third Era."

Badass immortal orc librarian wizard? Or something completely different?

Not enough information to guess really.

While it probably boils down to semantics (without people like him), I love the idea of him prolonging his life to expand and protect the book collection he started way back in the Second Era. He does remark that his books are "everything that matters" to him.

Headcanon accepted.

Dude is ancient.

Living proof to the theory that Orcs get grumpier as they age.

The College probably just staffs orsimer as head librarians, as whenever an orsimer likes books, they FREAKING LOOOOOOOOOOOOOOOVE BOOKS and will do ANYTHING to protect them.

Urag gro-Shub is probably just the latest in the line of orsimeri librarians of the College.

Necromancy's good for more than just throwing skeletons around, ya know.

Oh yeah, forgot that most orsimer mages we seem to encounter and hear about are necromancers

He is a mage. But, I really have no idea.