If Open Middle math the website is the WHAT then open Open Middle Math the book is the HOW. The website provides so many wonderful problems as a resource for the upper grades math classroom, but the book thoroughly explains how to introduce students to these types of problems, how to prepare for responses, how to carefully select responses to share, and how to make your own open middle problems.
What a fantastic companion to the website. I highly recommend buying this book before diving into Open Middle Math problems with students, or if you have had a bad experience with student response to OMM (giving up too soon, feeling frustrated, etc.) in the past and want to give it another shot. It will be well worth it.