First off, this is a small book. 72 pages including a glossary. I was able to read it on a plne ride from Louisiana to Denver. Like the tagline says, I hope I never need this book.
As the title suggests, the book is about making Java more functional. On my team we spend half our time in Groovy and half on Scala. I suppose we have done similar things in Groovy. The support that Groovy has for closures makes it easier than it would have been in Java.
He also covers what is coming in Java for more FP features like closures. The syntax looks clunky but I could see living with it if I had no choice.
I found the book useful in reviewing and appreciating where I'm at. How things were in Java and how nice things are in Scala. Even if you don't need to do functional programming in Java right now I think this book is a good read. If my current gig fell apart and I had to do Java fulltime then I would grab this book to keep my sanity.