Full disclosure: I got a free copy of this book for review purposes.
It's a solid book that goes through a lot of important stages of using Vim -- from basic buffer movements to working with large projects. Vimtutor limits itself to general-purpose movements and manipulation, and the built-in help is an excellent reference, but a book like "Mastering Vim" is a good missing link for learning what neither of the two covers -- workflows, plugins, and good practices for a modern Vimmer.
In some cases, I wish there were more references to the docs and explanations of built-ins. Most plugins looked into are also quite popular, so if you've already got a solid Vim workflow and have been keeping up with blog posts and feature announcements, you might not get a lot out of the book. There's a good intro to Vimscript, but, as the author himself says, it's just an intro and a book like "Learn Vimscript the Hard Way" might be a better choice if you're interested in it.
I'd strongly recommend this book to relative beginners who either don't know Vim at all or only have a passing knowledge of it from ssh-ing into servers and using it by necessity. I'd also suggest it to "old-school" Vim users who haven't been keeping up with modern plugins and Vim 8 features. They could find a good guide to refreshing their configs and picking up new workflows.