Update 10/5: I was unhappy with how much filament was being used so I simplified the design so it prints flat and in fewer layers, and added a hex pattern to the middle to further cut down on filament use. It could probably be hollow for extreme minimalism but hex pattern all the things i guess? I also cinched the sides a bit to allow for sneaking around supports on some bookshelves. The tool retains the same 100mm depth as before. STL and 3MF files provided for those that wish to tinker, remix, or whatever makes you happy.

My wife wanted her bookshelf to be much more presentable, so I made this simple jig. One end indexes against the edge of the bookshelf, and the other end is used as a fence/stopper so all books can be aligned at the same depth, regardless of the size of the book.

Sets books to a constant depth of 100mm from edge. 3MF and STL files provided if dimensions need to be altered at all.