Also known as 'flick-books'.
Picture books, with incisions into the front edge and revealing different pictorial worlds depending on the position of the thumb flicking the pages over.
As early as 1550, Geronimo Cardano (1501–76), an Italian physician and mathematician, writing in his De Subtilitate, was already trying to describe these miraculous little books and their hidden secret.
In the German-speaking world it was Daniel Schwenter (1585–1636) who first explained this apparent magic in his Mathematische und Physikalische Erquickstunden (Uplifting Mathematic and Physical Moments).
In literature, this device was described by Johann Christoph von Grimmelshausen in his Seltzame Springinsfeld (The Singular Life Story of Heedless Hopalong, 1670) as a ‘wondrous mirage bag’. In demonstrations at annual fairs, the public paid to blow into the book, and the showman would reveal to them their inner thoughts. This naturally led to the term ‘blow books’.