Books were hollowed out with a bandsaw, leaving intact the two opposite end covers of each room set. I used 7 or 8 books for each room. Leaving whole one cover at each end of the set gives you access to the interior of the rooms from either end. For these rooms, I then lined the interior walls with mat board and made a floor out of Gatorfoam Board (it is constructed with a center of foam core sandwiched between two veneers of wood). As I made more of these book boxes, I found it much easier to build a box separate from the books, finish it, then insert it to the hollowed-out books.