We have covered a lot of ground in this book. This is by no means everything to be experienced with Spark. Spark is constantly growing and adding new great functionality to make programming with it easier. Projects integrating with Spark seem to pop up almost daily. The Apache Mahout project has done some integration to bring more machine learning libraries to Spark. Projects Jupyter and Apache Zeppelin bring Spark to web notebooks.
This book will continue to evolve as things change or important new content is created within Spark. Bookmark things that you feel are important to you. If you didn't work through all the examples, come back and reference them later when you are ready.
There is also an Apache Spark Cheat Sheet available in HTML. This cheat sheet covers the most commonly referenced functionality, including links to the latest API documentation for each method covered. Hopefully this will help you become the Spark expert you want to become in no time at all.
Finally, if you haven't already, I suggest heading over and trying out the MapR Sandbox. It is a simple way to get running with Spark. The MapR Sandbox provides tutorials, demo applications, and browser-based user interfaces to let you get started quickly with Spark and Hadoop.
Good luck to you in your journey with Apache Spark.