Great resource, but a bit outdated now, so while back in the day it was probably good, nowadays there might be better options to have a more in-depth knowledge of phpunit. However, it's short enough to give it a chance and get up to speed quickly.
It's nice and easy in the first half of the book, but it gets more complicated by the end. Some tests return errors probably because the book focused on an older version of PHPUnit. The formatting for the example codes makes it difficult to copy-paste and some lines go off the page. The official PHPUnit documentation is written better, so you're better off with that.