How outdated is “HTTP: The Definitive Guide”?

I’m considering buying HTTP: The Definitive Guide with the goal of properly understanding HTTP and related technologies. I’m wondering if—13 years after its publication—the book is still worth buying.

To help me make a decision, I’m looking for input on two questions:

1. What important changes have happened in the world of HTTP that the book doesn’t cover? (For bonus points: what resources are recommended reading to catch up?)

2. Are any of the sections not relevant anymore at all?

Check the book’s table of contents to get an idea of what’s covered.