The people developing ExtendJ have published a couple of articles on various aspects of the compiler or its development. These are listed here.
An OOPSLA 2007 paper describes the basic architecture of ExtendJ (then referred to as JastAddJ):
The Java 7 module has been documented in a Master's Thesis, and a later conference paper:
The development of the Java 8 module is documented in this Master's Thesis:
These are articles that use ExtendJ to build language extensions for Java:
Over the years a number of research groups have used ExtendJ in their own programming language or compiler research. More publications that use ExtendJ/JastAddJ will be listed here soon.