This is quite the update - very sleek. My favorite new feature is the "show on hover". Just put your mouse where the icons should be and the appear like magic!
It really cleans up an already improved menu bar in macOS Big Sur.
We use HelpDocs.io (love it) and they have a "set article to go stale" option. I haven't used it, but it seems like a nice reminder to re-evaluate or delete the page.
What a great project. I have a very similar book (1800 Mechanical Movements, Devices, and Appliances) and have a blast flipping through it. It is great to see the illustrations come alive. Great great job!