The fold-to-fold dimensions of organ books are not of great importance.
Many makers had their own page length (Gasparini had the largest,
I think).

Most European organ books now have a fold-to-fold length of 16
centimetres and so have the Carl Frei books.  The main reason for
this is that the cardboard is made and delivered in rolls 33 cm wide.
You always loose some material in cutting it to perfect rectangles
after folding.

Hans van Oost, Netherlands