I particularly liked the chapter on errors. It has quite a bit of code that I copied into my project as a starting point.
When it came to testing, I liked that it acknowledged the need to do integration testing, that is, testing the API with a real database backend. I personally don’t buy the mock everything custom of the Java world as bugs can hide in the integration and these days it’s so easy and fast to test against a real database that I don’t see any value in mocking it out (if you do, the book covers that too).
It has a small section on writing clients using RestTemplate which I wish it was bigger but I understand my needs in this regard are not that common. Particularly, there was no mention on how to deal with the validation errors scheme introduced earlier when using RestTemplate.
All in all, I like this book. For me, computer books should either be short or comprehensive (not padded) and this one is a good example of a good short to-the-point book.