An OrganScore book is an organ score with no page turn problem. This example of  Volume I of Bach Complete Organ Works demonstrates the benefit of using a large format for the score. There is nothing better than having a look inside the book: for an insight of Volume I of J.S. Bach organ works, flip through a low resolution pdf and check where the few remaining page turns have been set.

NB. The resolution of the “flippable” pdf here does not reflect the resolution of the real score!