What you need for this book
You should acquire Java Development Kit version 8 or higher for your operating system to work through all the examples. This book discusses the Oracle HotSpot JVM, so you may want to get Oracle JDK or OpenJDK (or Zulu) if possible. You should also get the latest Leiningen version (2.5.2 as of the time of writing) from http://leiningen.org/, and JD-GUI from http://jd.benow.ca/.