The ultimate book gadget
This may just be the most profound book gadget in history. The image from 1588 shows the book wheel invented by the Renaissance engineer Agostino Ramelli. The idea was to give a scholar the opportunity to consult several books at the same time, each remaining open at the page that was consulted last. The version in the image contains about twelve books, all well balanced as the giant wheel turned. In the age of small desks, this device was a revelation. It encouraged people to craft studies in the modern sense of the word, using a variety of sources that were discussed simultaneously in a scholarly treatise. I have sat behind a smaller version surviving from the 17th century (placed in a 17th-century library) and it is quite an overwhelming experience to see all those books spin before your eyes. Knowledge overload.
Pic: The Diverse and Artifactitious Machines of Captain Agostino Ramelli, a book of engineering designs from 1588.