I love the spandex idea that Spectrun used, but you can also take fishing line and run it through one of the sides of the empty bookcase and tie it to the other side, now insert the books so that they have the fishing line strung behind them ( the fishing line is in back of the books).  When it is time to make books fly, have a stage hand pull the side fishing line through the hole in the side of the bookcase.  The books will fly.  If you want even spookier, run the line under the books , so now the books rise and fly off the shelfs.  You can even be selective by stringing the line behind  a few books, but in front of (or through) others.

I remember doing Blythe Spirit years ago, I think I remember a secret entrance we made next to fireplace.  The wall with the fireplace was angled and the wall that was on the upstage side was also angled--but parallel about a foot.  The audience could only see continuous wall, but an actor could squeeze through.  Its a sightline trick.

If you want the mantle to crack upward, but not come apart, you can use a springloaded hinge that pushes up both sides of the mantle when 'broken'.  The key is to keep it in place when not 'broken'.  To do that, you have a dowel that you remove (Drill hole in flat through to one side of the mantle. Insert dowel to temporarily hold it in place).