I just don't see why Ven or Vanitas would be in Eraqus' study. Ven, I could understand, being the man's apprentice. But Vanitas? If Eraqus is anywhere near being as good a Keyblade Master as he should be, he'd know dark powers when he sees it. I also suspect MX would want Vanitas kept secret from Eraqus. I suspect it's a very Dumbledore/Voldemort relationship, with Eraqus being less like Dumbledore and more like Mitsurugi from Soul Calibur.
And it's not far fetched to think that it could be Ven's room. I'm pretty sure Terra, Aqua, and Ven all live in the Land of Departure. And as an apprentice/student, I'm sure all 3 of them would have TONS of books in their rooms. It's looking like wielding a keyblade never used to be as rare as we once believed, so I wouldn't doubt it if there were books written on the subject. The telescope could also be for studying the sky, identifying worlds and correctly placing them on a map of the Sea of Worlds.