Book of Storing

After you attune to this 1-foot square 6-inch thick book it hides its true nature to anybody else, appearing to be a mundane diary. When you open it however, the book reveals a Tiny storage compartment. The storage space is the same size as the book, and because this item doesn’t function as a pocket dimension it can be safely used to store such items (such as a __portable hole_ ).

Source: Level Up Advanced 5e · OGL-1.0a · 5.1