One of the simplest hidden compartments I know. As an avid book reader, my house id filled to the brim with books, some of which I've inevitably grown out of.
Thick, hardback books works best to make this, but most books will work.
Step 1: Equipment
You will need:
A book you no longer want
A pen knife or Stanley knife
Step 2: Step 1
All the following measurements depend on the length and width of your book.
Pick a page,30-40 pages away from the start of the book.
Draw 4 lines, parallel to the edges of the page, a few cm away from the edge of the book.
Step 3: Step 2
Using the blade, cut along the lines. The sharer the blade is the better because it cuts more pages at a time.
You may need to sharpen the knife every now and then, as paper dulls the blade.
Carry on cutting along these lines until you reach 30-40 pages before the end of the book.