Practical VIM

I used to stuff external packages into my .vimrc files without any consideration or understanding of anything. This book made me realize there are so many powerful built-in functionalities right in your default Vim: registers, marks, global, etc. And you can be pretty efficient with them: eliminating abundant motions, lazy thinking before moving or editing, one terminal with Vim to rule them all, etc.
The most important lesson: "Know the saw, then sharpen it"!

I actually read it from cover to cover, which I hadn't meant to do, but it was very easy to consume and, as an experienced Vim user, I got way more out of it than I thought I would.

Fantastic, in very few pages I learned more useful new things about vim than in 10 years of just messing around with it.

I know, it's still beta, but so far it makes a good book to just look some stuff up, but so far it's not good enough to start with the book as a complete vim-beginner. To start from scratch you might want to have a look at this blogpost which sounds quite reasonable.