Spring is a widely used application framework. However, configuring a Spring application can be pretty painful. Spring Boot heavily simplifies the development by embedding a lot that is needed. It provides, for instance, a servlet container and support for an easier configuration. These are only two essentials of Spring Boot
Spring Boot in Action by Craig Walls is a good introduction. The author starts by highlighting what Spring Boot is and what it is not respectively. After that, the reader can get started learning Spring Boot in a nicely structured way. Throughout the book, one application is developed while various features of Spring Boot are explained and used.
One thing I like much is that the author does propose different build tools (maven and gradle), so that the reader is not “forced” to follow one way to succeed completing the different milestones.
In the end, the author presents even different approaches to deploy the sample application to different cloud solutions. Again, the reader is not “locked” to one solution.