Free, online, open-source book on web performance - Book of Speed

Stoyan Stefanov, a web performance expert, Facebook engineer, ex-Yahoo, smush.it creator & YSlow 2.0 architect, has released the draft of Book of Speed for free & public access. Currently 5 of the planned 9 chapters are online.

In his blog post announcing the release, he also mentions another free, online JavaScript resource by Marijn Haverbeke - Eloquent JavaScript.