Well, the OP specifically asked for texts on the fundamental subjects of CS as they relate to web development.

Since you bring up a good point though, what would be useful open source projects to contribute to if you're specifically interested in exploring deeper issues?

For myself, I remember learning about networking (and especially coding the network layer in C) and also a bit about crypto by tinkering with World of Warcraft unofficial open source servers two years ago. Wish I had the chance to contribute something back before real life took over again.

I don't think you need to necessarily choose one or the other. I've gotten a lot out of texts that give you practical assignments that get you thinking about what you just learned.

I feel that learning in general is an iterative alternation between theory and practice. At the end of the day if you get both sides of that coin, you're doing fine.

This, one thousand fold.

A lot of projects on the "Get Involved" page or whatever, have a list of "prerequisite' knowledge. Don't let this intimidate you, often playing with the source is a much better way to learn about the subject than trying to research that topic, feel you understand it, then looking at the source.