As I find useful resources to AKKA I will list them here.
Akka Documentation. The documentation is a great place to start.
Slideshare Akka Microservices. This slide series shows the related DDD concepts and how they relate to AKKA and Scala.
Here is a link about why you shouldn’t create too many akka actor systems in your application.
Akka anti-patterns: too many actor systems
Learning Akka by Jason Goodwin. I like this book because it is small and to the point. At about 250 pages this book jumps right into AKKA, it doesn’t tell any stories and the examples are concise and to the point. This is also a great starting book for Java developers looking to get started with Akka because the author shows how to do everything with Java then with Scala. What immediately grabs your attention as you are reading this book is… how much more code it takes to do everything with Java. <– And that is why I don’t Java anymore. Java is like writing a novel to describe a page. This book mostly covers the basics of AKKA it doesn’t go into depth about all of the AKKA features and abilities. This book is just a nice, clean, overall introduction to AKKA with Java and Scala. If you are new to AKKA this should be your first book.
AKKA concurrency by Derek Wyatt. This is an older book, however it is still relevant. This book is kind of long at over 500 pages, but it digs into how AKKA actually works. What is nice about this book is it starts with an example and follows through with iterative changes as the book proceeds. The example is of a plane and flight crew and how all of the parts ( actors ) of the plane and crew interact to make the plane work. This book is more advanced and in depth than Jason Goodwins book, but I feel it explains the reasons behind what you are doing with AKKA. I’d read this book after reading Goodwins book, as it digs a little deeper into things giving more info on how AKKA works.