I have now finished the testing chapter (Chapter 8).
This was rather frustrating to work with since you have to enter a lot of code before you get to run the tests. This means that it is very easy to make a mistake and spend a lot of time tracking down the issue.
This chapter gives a lot of details on testing a Phoenix application at different levels.
Beware some of the later parts of the chapter – it has a trick problem that is intended to fail and explains on the next page. This can be frustrating if you had previously made a few typos and were running each test as you go.
Developer Rants 