Hello Dirk,
There are few reasons behind breaking the "Beginning
Java 8" title into three books:
1. Over the years, Java has become very vast. In every release, you see new features added to Java. This is a 2200-page book. No readers would like to carry a book that weighs over ten pounds. So, the big size of the material as one book was one of the reasons for the split.
2. The series covers all topics in core Java. Not all types readers will need to read all topics covered in the book. So, why to pay for the material you won't need. For example, a beginner can start with the title "Beginning Java 8 Fundamentals". An intermediate-level developer may want to pick up the title "Beginning Java 8: APIs, Extensions and Libraries".
The convenience for readers in using the material was the consideration in breaking it into three books.