This idea may be already out there but I came up with it (the middle "hidden" safe part- Nicole gave me the rest of the idea) on my own. It's a twist on the old Book Safe concept. Nicole (my girlfriends sister) gave me the idea for using multiple books to really hide something in a sort of false bottom- only false middle!
Nicole is a creative type and makes all kinds of cool artwork from ordinary things, some paint and even a cutter. She's been on this really cool project (also not new) of going to the dollar tree to buy up any cool thick books that are hard cover with a dust jacket. They have plenty.
Removing the dust jacket the books cover beneath is the main (other than thickness of the book itself) event. Gold leafed titles or just cool book names (they got a bunch of these with intriguing names!) and she's all set. 4.00 and 4 books later (no tax here in Oregon) and she takes these home to create a hidden treasure book (old school concept pictured above).
You just leave a border, cut out the internal guts of the book and use rubber cement/glue to bind the insides of the hidden safe infrastructure this creates. For some of these she actually glues the books themselves together into sets and cuts all the way through the first book and into the next one to create a nice well (deep safe). This can be useful to save the second books cover cut out as you can put it in for a false bottom effect.
You can also cut this slightly bigger than the hole in the first book (before gluing the books together) to create a truly hidden secret safe- you hide something cool in there and only cut through to the middle of the book leaving the back of this book uncut. You then replace the books cover (cutting into the second book you also leave a border to glue the cover back to "seal it" permanently (for now).
using 3 books for this total, you glue another book on the back end of the middle book and cut through to half to 3/4ths of the way in (and don't cut through to the middle book).
What the person getting this "gift" will think is that they are getting a 3 book set with two of the books having a secret compartment with their actual gift in either side (use this as a new type of Christmas gift "box" and wrap it for an intriguing gift).
You tell them after they open it up that both of the outside books have a gift in them, the middle book is just "for show". You hide something valuable in that middle book and secure it so it won't move around (cologne, money, etc... I prefer money i.e. a gift card visa works great or folding money is cool too).
You can give this gift and make them think they figured it out completely but later, say on their birthday, you can let them in on the final hidden compartment! Make sure whatever you put in there is wrapped up in something that will stop it from moving around! It's a great way to give a "gift that keeps on giving"!
You may want to glue a bunch of pages between the end of each sides hole/well and the backing (entry into the very center) to throw them off.
The ways you can get cleaver with this is endless. I had the notion to build one of these for my kids (when I have them) and give it to them as a "cash" gift. Putting money in it but hiding money in the middle too so when they one day need more money but spent what they got, I can point them to the new found wealth they had all along and didn't know about it!