The LEGO Contraptions book is great! About six or so years ago I got it for my 12yo son, and I honestly had as much or more fun with it than he did. I had never played with gears, so learning how they interacted and how the ratios worked, was enlightening to me. I ended up buying a set of LEGO Technic Idea books [1], then a massive crane set with motors, etc. just to keep playing with it all. Seriously, the amount of money I spent as a result of that one book is obscene.

1. https://www.amazon.com/gp/bookseries/B01N9XHU2M/