Web Browser Engineering
It’s heavy on computer science, but this is a fascinating endeavour. It’s a work-in-progress book that not only describes how browsers work, but invites you to code along too. At the end, you get a minimum viable web browser (and more knowledge than you ever wanted about how browsers work).
As a black box, the browser is either magical or frustrating (depending on whether it is working correctly or not!). But that also make a browser a pretty unusual piece of software, with unique challenges, interesting algorithms, and clever optimizations. Browsers are worth studying for the pure pleasure of it.
See how the sausage is made and make your own sausage!