Java 8 with its introduction of lambda expressions is the biggest change to the Java language since it was created some 20 years ago. No escaping from it - if you want to stay relevant as a Java programmer you will need to know about lambda expressions and it is certainly worth the effort to really get to grips with the new semantics and paradigms that Java 8 delivers.
Naftalin's book is actually the 3rd book I've read on the subject. The first 2, whilst reasonably well written, didn't really deliver much more than I could have learnt by reading the numerous tutorials out there on the web. Any Java 8 tutorial will explain how to use Lambdas in your code and if that's all you want then you don't really want to buy a book at all. When I buy a book I expect a lot more than that. I expect a beginning, a middle and an end. And to this expectation Naftalin delivers beautifully. He explains the rationals and motivations of Lambda expressions. Are they just a sprinkling of syntactic sugar? No, he explains, they are much more than that. He sets the scene, as is hinted to in the title of his book 'a multicore world' and explains how Java needs to adapt to continue to hold its own so that it will still be one of the most important languages for the next 20 years.
I've been using Java 8 for the last 6 months and am fairly well aquatinted with the syntax but when I read his description of the syntax I could only wish that I had this book 6 months ago! Naftalin has clearly put a huge amount of effort and consideration into how he delivers his ideas, which is why, perhaps, it wasn't one of the first books on the market to greet Java 8 as it appeared in the early part of last year.
Some books go off on a tangent trying to introduce their readers to functional programming through lambdas. I think that is a mistake. I've been a Java programmer since the language emerged and have no wish whatsoever to change my programming style into a functional style. If I would want to do that I would use a functional language like Erlang etc. I want to continue using Java but to introduce some new concepts that can be delivered through Lambdas. The declarative nature of the Lambdas are extremely nice but we don't have to throw out the proverbial baby with the bath water and ditch everything that is good about OOP. I believe that Naftalin shares this opinion as is evident by the way in which he introduces as to practical uses of lambdas in our code.
In conclusion, I would highly recommend recommend this book to any Java developer that wants to get a real understanding of lambdas. The book is extremely readable, not particularly long (all the best books are short) but will tell you everything you will ever need to know about the subject. If you digest all the information in this short book you will be a far better programmer than someone who has just read a few tutorials on the web!