Grab any Horse and Carriage to Winterhold. Once at Winterhold, turn right immediately. Travel Southwest from Winterhold to a location called "Sightless Pit". Just above Slightless Pit a little higher on the mountain (not much higher) you will see a alter with a skeleton on it, beside the skeleton is a book called "The Doors to Oblivion".
Pick up the book, and kill the skeleton on the table, Two other skeletons will rise around the alter, kill these as well.
Return to the Skeleton near where you picked up the book, and loot it's body, take everything, and loot again, you'll notice the book on this skeleton continually re-appears, giving you an unlimited supply of "The Doors of Oblivion" worth 50 gold each. Every time you view the Skeletons inventory, even if you do not remove the book, adds 1 book to the skeletons inventory.