There is a wikibook that describes the internals of GCC and GEM, an extensibility framework.Your internals documentation looks pretty good, so I have made a link to it from gcc.gnu.org/readings.html. Thanks,
It describes AST part of GCC source code. We would like to ask developers to work on the intermediate representation and backends. When the book gets comprehensive enough we will generate a PDF from it for a convenient distribution.
Alexey
