CPU is one of the places where you can hide things such as personal diaries, books or other small things. Its a good hiding place because no one would try to check a computer case.
Step 1: Hide Things Inside a CPU Case
A book can be hidden in the CPU case without damaging the CPU.
Open the CPU case and analyze whether you have physical space enough for the object to be in it. Although the large amount of space is for cooling the CPU, a book or any other small object of a small volume, is not going to affect it unless or until the object is blocking the air vent or the air cooler or the object is too large.