I think a simple drywall saw would make these "cash and carry". I'll stick with the hollow copy of websters dictionary on the bookshelf to hide my pearls, emeralds and rubies in.
The whole idea here is that the thief would never know that the receptacle was actually a safe. Once the "cover is blown", as you said a few strokes with a drywall saw and away you go.
Of course, once a thief knew about your hollowed-out dictionary, taking a book off a shelf is even easier than sawing out a bit of sheetrock, right?