I've already completed this dungeon but got sent back to retrieve a book for Urag at the College. When I get to the end of the ruins, Vokun's body is still there, but there's 2 locked gates at the end and the book is behind one of them. I've looked everywhere and can't open the gates? Please help.

UESP says:

Bugs

• There is a bug related to a Radiant Quest that says that the adventurer has to pick up a book for the librarian of the Winterhold College in High Gate Ruins. The book is situated in the room behind Vokun. If the adventurer killed Vokun on a previous run, then he will no longer have access to the room behind the throne room.
  
• (PC)This can be avoided by resurrecting Vokun(not his ashes) using the console and killing him again.

Hope youre on pc.

I'm on PS3. Thanks for the info. Good thing it's just a miscellaneous quest line and not a main quest or guild quest.