A trouser book is a very simple book that can be made from a piece of A4 paper or larger.
Try to be accurate and crisp with your folds.
Fold your paper in half, open out and fold in half the other way.
Open it out landscape in front of you and fold both sides into the center line.
Your paper has been folded into eight sections. Turn it portrait way and cut up to the gusset of the trouser (where the scissors are lying) in picture below (left) you will end up with a paper trouser shape like blow (righ).
You then fold this up into a concertina, you have to reverse some of the folds to achieve this. The result is a small concertina book with a central secret section.