For a pretty good introduction to Spring Boot 2.0 you may want to look into Learning Spring Boot 2.0 - Second Edition by Greg L. Turnquist found here
https://www.packtpub.com/application-development/learning-spring-boot-20-second-edition
In this book you create two micro services, where one service talks to the other service.
You are also introduce to the following concepts that you create projects for:
Spring Reactive Stream ProgrammingSpring Boot: HystrixSoring Boot: Web SocketsSpring Boot: Config ServerSpring Boot: Eureka Server Spring Messaging using RabbitMQ with Spring AMQPIn the end you have more then five different Spring Boot projects running at the same time.
This book teaches you enough to be dangerous.
However this book does not teach you exactly why you would want to use either an XML config or an annotation or what either of those do.
There is a listing of books and resources found here
https://coderanch.com/c/books along with
book reviews.
I suspect that there could be one or more books regarding Spring found there.