If you'd like to get started with Ajax, I highly recommend Ajax Construction Kit by Michael Morrison. It's terrific. Morrison's writing is clear, strong, and informative. The sample projects are practical applications that make great starting points for real-world applications. He thoroughly explains the stuff you need to know and doesn't get bogged down in the stuff you don't. Plus he includes just enough humor to keep things light and fun.
Stories by Brian Tanaka
I like screencasts. A well-made screencast can explain complex concepts more effectively than a written document. Watching a screencast about a topic you're trying to learn is like having a friendly expert by your side, showing you what you need to know. Recently, I stumbled upon PeepCode's screencast about Git.