“The Rust Programming Language,” is an introductory book about Rust, with multiple editions.
The source for all editions lives on GitHub. Please open issues with any questions, concerns, or tweaks.
If you're not sure what edition you want to read, Go with the second edition, as it's the most polished.
The second edition is still receiving some minor edits, but is effectively complete. It will be available in dead-tree form through NoStarch Press once these final edits are complete.
The second edition is a complete re-write of TRPL, from the ground up, and is therefore very different from the first edition. It's also based on Rust 1.21, and so may be missing features that landed in stable after that release.
You can find the second edition here.
The first edition is completely frozen, but is still included in the documentation for historical reasons.
You can find the first edition here.