TL;DR If you already read a little about Kanban, you can skip half of this book.
Visualize your actual workflow using stick-it notes on a board with columns representing stages of your process. Lower the times it takes to a workitem to be done/shipped/deployed (lead time) and increase throughput. How? Stop starting, start finishing and limit your work-in-progress (WIP). How do you limit your WIP? Setup constraints on WIP per team member, per process stage (columns on board) or per work classes (lanes/rows on board). This is basically what you learn in the first third of the book.
The book is rather long with lots of repetition of basic principles. I've realized that I might not be the target audience when the authors started to explain a simple ratio equation on multiple pages with the words: "Eh, what?". On the other hand, if you are non-technical person looking for an easy-to-follow and comprehensive guide to start using Kanban, grab this book. In overall I've learnd all I wanted to know about Kanban and kudos to the authors for the chapter about drawbacks of using Kanban.