There are numerous web articles that address the new features of Java 8. The advantage of a book on Java 8, is that it is able to bring structure and a big picture perspective that is missing from isolated articles. In addition, more detailed and often better explained examples can be found in a book.
Yes, there have been many new features added to Java 8 and a lot of them are not necessarily simple. The book does, in some places, contrast the newer approaches provided by Java 8 to earlier techniques. For example, the use of lambda expressions is a lot easier to use than anonymous inner classes and this difference is contrasted in the book.
What actually surprised me was that this book was shorter than Richard's Java 7 New Features book! I consider the changes in Java 8 to be more significant. But as I read the book, I realized the Java 8 changes are more focused so the coverage was still good.