Very good point! You don't get "how to write a browser" books in the same way you do with game engines. I wonder if it's because game engines are just more appealing? And also, you can build your own cut-down game engine and that's still useful, whereas there's a very high threshold of functionality needed before a browser is useful.

They're actually similar in some ways -- there are a handful of browers and a handful of AAA game engines, and they're all either open source or at least licensable source.