In the castle in the underground city, head to the door off to the left (but not all the way to the left), it should be a library. You'll find it in a basket in the bottom right corner of the room.
Since I've already collected it, it doesn't show up, but it's in the wrecked barrel on the bottom right part of the screen. Also, my earlier comment has the wrong location, and I have updated it with the correct information
