[I've also written a longer review of the book in my blog. You can find it here.]
I had already used the Headspace meditation app for a year before reading the book. Still, the book was definitely worth it! For example, the Headspace book goes deeper in the theory and reasoning than the app. The clear explaining also helped me correct some misconceptions I still had about meditation.
The Headspace book does a great job of teaching mindfulness in a very easy-to-understand way. Instead of giving the reader an information overload, the book has a clear focus on the most important information. It also helps that Andy Puddicombe is a very engaging and entertaining writer.
If I'd need to recommend just one book for someone who wants to learn mindfulness and meditation, I'd hand them "The Headspace Guide to Meditation and Mindfulness".
Ps. "The Headspace Guide to Meditation and Mindfulness" and "Get Some Headspace" are listed as different books in Goodreads, but make no mistake: "The Headspace Guide..." is just a re-branded version of the exact same thing.