Full disclosure: I was a technical reviewer for this book
This is just a simple but rock-solid book on unit testing in Java 8. I wouldn't let the title fool you - it doesn't seem to be a big Java 8 book, in fact it doesn't really even seem to leverage the features Java 8 introduces very much. I think mostly it's a book about Unit Testing in Java, and Java 8 happened to have been released at the time, so it got thrown in the title. The code definitely IS Java 8 (not 7), but I wouldn't say that it's a book all about how Java 8 introduces new features, and here's how to unit test code that uses them, which is kind of what the title implied to me.
Mostly the book seems geared to people who are brand new to unit testing, maybe people who had never written a unit test before. The different mneumonic devices (CORRECT, FIRST, etc) underscore the targeting of newbies. However, it does NOT seem like it is a book for people new to Java itself, or even Java 8, since very little explanation is provided for any of the code aside from the unit tests themselves and why they are the way they are. If you're a big unit tester, you might find a lot of the book doesn't provide you with anything new, but honestly I actually recommend reading it anyway.
Part I is going to be familiar to anyone with background unit testing - it's just the basic mechanics of testing itself. Part II however might give even unit testing veterans some stuff to think about. It's basically all about how to write effective tests that really stress the system - and it introduces a number of mnemonics to that end. A little cheesy but I actually learned quite a bit here, and was surprised at how useful I found it, given how much experience I have writing tests.
When I first started reading Part III, all about the overall design of your code, my reaction was thinking, gosh, a lot of other books cover refactoring... it seems out of scope for a book on testing. I was very skeptical, and I didn't understand why it was included. But once I kept reading, I saw the point, and saw how using mock objects were going to allow us to refactor the book's primary example code into something more testable, and better-designed because of it (it even fixed an issue I had with the code when I first saw it, the side effects in the scoring method). By the end of the section, I was fully on board, and to be honest an entire chapter about refactoring tests was simply amazing (and easily my favorite chapter). To be frank, these later chapters are worth the price of admission alone for the book, I can't remember any other single book that covers refactoring to improve quality of tests themselves (especially in a step-by-step fashion). I can't even remember a book that covered testing multithreaded code, and for it to just be a chapter at the end of a book on testing in general was a real treat. This book covered absolutely everything, mock objects, refactoring, TDD, SOLID, and of course the basic principles of unit testing in general.
I was pretty impressed with how thorough and useful parts I and II were, but I was blown away by III and IV. Simply put, this is probably my favorite book on unit testing I've ever read. If you're new to unit testing and use Java (even if it's not Java 8), this book is a must-read. If you're a unit testing pro, there might not be that much for you here, but I still recommend giving it a read anyway and just skipping bits and pieces. Parts II and III in particular have a lot of valuable insight to share.