It does kind of depend on the money. For instance, taping it to the top of the door would work if 1) it's notes/bills and 2) it's not a lot of notes/bills. Can't be too thick or you may not be able to close the door.
I personally never had a problem with losing money or having people steal it. When I was young I had a glass jack-in-the-box bank. Stick the cash in and you can't get it out unless you open up the stopper. Meaning then that someone strong would have to do it. I'm the oldest of three, so my brother and sister were unable to get into it because well, they weren't strong enough to open it up. And breaking it doens't work..that's too obvious. By the time it broke and the neck came off, I was old enough to start keeping it in a purse. Same time though, my sister and brother don't steal money because they earn it also.
But I'd suggest behind something, like if you have a mirror (girls do) tape it to the back of it. You don't want to end up forgetting where it is, so you almost want it someplace you remember. If it's coins and bills/notes, you want to find like a hollow case of sorts. Perhaps look into getting like..a locked box. Stick the money in there, lock it up, and carry the keys on you. I have one of those as well, it works nicely because it's also got a little slot on the top where you can put the change and money in.
Don't have to hide the money if people can't get to it. :/ But if these are unaccessible, I like the book idea. I have a lot of books, so it'd be trouble to look through them all. Find an old hardback book and kind of make like a hole in the pages. See them movies where people open up a book and there's a gun or something inside? Like that. Cut a block in the pages, glue these together with watered down glue so they're stiff and not going to move, and then stick your money in. Homemade box that closes and fits nicely and blends in nicely into your bookshelves. Make sure it's a book you don't mind not being able to read again, but also a book that you would find in your room. Old HP book maybe?
The safest place though is 1) the bank (when you're allowed, because then you also get interest, but you want to have an actual job, like waitressing or working at the local store) and 2) on your person because then you always know where it is, assuming you don't lose your wallet/purse or have a hole in your pocket. There's really no 110% safe place though. Just gotta be able to trust that the place you choose is the safest and make it the safest.