If anyone wants a much more in-depth look at the concepts behind this, I can't recommend "The Goal" by Eliyahu M. Goldratt enough. It's a bit like The Phoenix Project, in that it's a novel about improving flow in delivery processes, but it's much, much better, and goes into way more practical depth. You'll learn about not just WIP, but floating bottlenecks, buffers, batch sizes, etc. that turn out to be universally useful concepts. The audiobook is especially good - easily the most practically applicable book in the field I've come across. Go read it/listen to it.
Less known but even more applicable to software is "Principles of Product Development Flow." I learned about that book from the talk "Architecture Without an End State" and it's one of the most thoughtful things I've read on the topic. It references "The Goal" but builds out its ideas even more, mixing in a bunch of queue theory and taking lessons from many other areas such as statistical economics.
Reinertsen's Principles is an amazing book. The way he combines lean, network engineering and all the rest to create an actually meaningful and coherent picture of product development is genius.
Author here. I loved "The Goal" even more than the Unicorn Project. Use the Theory of Constraints to identify your bottleneck and then limit WIP at the bottleneck or elevate it. By hiring, optimizing...
