As you probably know, whenever we add a new feature to symfony, we try to update the symfony book as soon as possible. So, the book is 'always' up to date with the trunk. That's great if, like me, you work on the latest trunk.
But if you work with the latest stable release, or if you still have a project that runs on the 0.4 branch, you are out of luck... until now.
You can now browse the symfony book for every symfony stable release.
So, if you work with the latest symfony trunk, browse the "trunk" book. If you work with the 0.6.3 stable release, read the "0.6.3" book.