I believe they could help you with your day-to-day GWT development so I decided to aggregate them all at this post.
So, here they are:
(NOTE: Please let us know if some of these tips are outdated so that we remove them from the list)
Get ready to program in a whole new way!
Functional Programming in Java will help you quickly get on top of the new, essential Java 8 language features and the functional style that will change and improve your code. This short, targeted book will help you make the paradigm shift from the old imperative way to a less error-prone, more elegant, and concise coding style that’s also a breeze to parallelize. You’ll explore the syntax and semantics of lambda expressions, method and constructor references, and functional interfaces. You’ll design and write applications better using the new standards in Java 8 and the JDK.