Interesting– you don’t usually hear the term “sketchbook” applied to things this old. I think back then paper was a more valuable commodity and wasn’t used as much for practicing and doodling, as some of these pages seem to have been. Or else those rough pages just didn’t survive, and were perhaps erased and re-used. This manuscript, which belonged to a German monk named Stephan Schriber, is a mix of illuminated manuscript pages in progress and other pages that just seem to be notes and practice sketches.
See more at The Illuminated Sketchbook of Stephan Schriber (1494) | The Public Domain Review.
Incredible that a notebook that old survived. Fascinating to think of who could have been the author/artist in old notebooks, what their lives were like, and if any of our notebooks might survive centuries too.