When it comes to learning a system, my favoured approach would be to just dive in, and look things up while I go.

I'd recommend grabbing a distro like Arch, something lean, which requires a lot of hands-on and has pretty great documentation, and just get going.

As far as I know, there is no such thing as a "definitive book about Linux", since there are so many topics to write about, it would be impossible to fit it all into one book.