These tutorials are good enough to get you started but obviously, it doesn’t cover everything that you will get in a book. That’s why I am providing 3 good Java 8 Books that you can read and upgrade your skills.

Note that these books only cover Java 8 related features, if you want to learn java properly you should also go through some of the books mentioned in the below posts.

1. Java SE 8 for the Really Impatient: A Short Course on the Basics
   This 200-page book is all you need to get started with Java 8. It covers Lambda Expressions, Stream API, Date Time API, Concurrency Enhancements, Nashorn JavaScript Engine, Other improvements and a short brief on Java 7 improvements. These topics are covered in brief detail to help you get an overall knowledge of these features.
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3. You can get “Java SE 8 for the Really Impatient” book from below stores.

   Amazon.com Amazon.in (India)  Flipkart.com (India)

4. Java 8 in ActionJava 8 in Action is the book I would recommend if you want to learn Java 8 features in more depth, it covers Java 8 features in more detail and it’s a good read for experienced developers.
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   Some of the best parts of this book are Java 8 and Scala Comparison and Lambdas internal implementation.

   You can buy “Java 8 in Action” book from below stores.

   Amazon.com Amazon.in (India)  Flipkart.com (India)

7. Java 8 Lambdas: Pragmatic Functional ProgrammingLambda Expressions are the most useful feature introduced in Java 8. This book covers lambdas in more detail than any other book. It also shows you how to utilize them in Stream API for parallel processing.
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   You can buy “Java 8 Lambdas: Pragmatic Functional Programming” book from below stores.

   Amazon.com Amazon.in (India)  Flipkart.com (India)

All these books are highly rated and will help you in upgrading your core java skills to Java 8. Note that these books are dedicated for Java 8 only, so a good core java knowledge is important to get the most out of them.