When the book's front cover is lined up in a certain way, it represents the title of the book.
Which of these five designs is the front cover?
If the book is not placed in a certain way, the cover does not convey the contents.
It's important to place it so that the front cover is displayed correctly.
It's hard to see the meaning protrayed by the cover with just one copy of the book.
Try imagining lots of books lined up together.
A certain pattern becomes apparent when the books are lined up correctly, but you need four copies for this to work.
Which way should they be lined up?
Continuing on from hint three...
When you line four books up horizontally and vertically, it makes the shape of one heart.
Answer for 'The Shape of Love'
The answer is '4'.