Ask HN: Great online reference books?

I just discovered Why's (Poignant) Guide to Ruby today, and it utterly fascinates me. I know I'm extremely late to the game on that one, but I've still never read a guide that's made me that enthusiastic about a language.

Are there any reference books that you absolutely love? I'm language-neutral: beyond the basic web languages (HTML, CSS, JavaScript/Java, PHP) I'm not familiar enough with any language to really love any one over others. Just: what if any online reference books do you love?