"Mastering Java 11 Ed. 2" is a book that helps experienced Java developers understand and utilize the latest features in Java 9, 10, and 11. It covers topics like modular Java applications, the G1 garbage collector, and performance testing with the JVM harness. The book also explores new features such as the Java Shell and the Process API. By the end, readers will have an advanced understanding of the Java platform and its recent changes, making them more efficient and productive developers.