There were also numerous hidden Jack Skellingtons added during last year's refurb. Unfortunately, most are nearly impossible to see under show conditions. One of the books on the table in the library near the rocking chair is titled "The Nightmare Before Christmas." Nearby, on the floor, is another book with Jack on the cover. There is also a large, plush Jack doll to the right as you're exiting the attic, all the way in the back of the scene. Nearby is a Nightmare Before Christmas snowglobe. There's also a hidden Sally in the attic, but I don't recall exactly where (near the first husband's portrait, I think).
Another miscellaneous HM tidbit: Though there appear to be many books in the library set, nearly all of them are simply a painted flat. Out of the painted ones, only three have legible titles. Two of them are The Art of Deception, Volumes 1 and 2 by Claude Coates (one of the main Imagineers for the Haunted Mansion). I don't recall the name of the other volume, but I don't think that it had any particular significance. Again, these are completely impossible to see under show conditions.