It came to my attention that after already shuffling a few books around, the one I eventually started reading for center square is over 500 pages. It's a key position and a book I've been wanting to read, plus I've read too far to stop now!
So, after looking at my options, I've decided the one I had lined up for Ghosts isn't that important to me and I've moved Bridge of Bones to the Ghosts square and looked through my Horror folder specifically looking for something short for Center square.
Mirror, Mirror will do nicely. 53 pages. Phew!