Just "the book" (v2, that is, http://rust-lang.github.io/book/second-edition/index.html), but I assume formatted better for ebook readers and printing. Some folks like physical books.

To add on to what Manish said, it's important to us that the core docs remain free and accessible, which is one of the reasons why it was great to work with NoStarch; they're super happy to keep the book 100% open source. Win-win.

In addition, the plan is to donate the proceeds to OpenHatch.

100% true