This one had me confused as well. The problem is, the book you need is not in the mirror, it's on the real world side of the mirror. This is not explained very well by Desdemona's site. I too spent a LONG time scrolling around the corner of the room. After giving up and returning to the real world, I decided to try it again, and found it almost immediately. I am guessing this is your problem. Hope this helps.
