If you bought the previous version of this book, the update is free. (I imagine I’ll do a more substantial update to the book next year.) It includes information about the new People interface, the revised sidebar, new editing tools, and more.
If you haven’t bought the book and use Photos on macOS and iOS, you may learn about some of the unexpected features of the apps. And people who are considering a dive into Photos and iCloud Photo Library may find it the perfect guide to getting up and running.