Ada 2005 reference and work on an Ada 2005 compiler
Annotated Ada Reference Manual (2012) -- Ada 2012
Annotated Ada Reference Manual (2005) -- Ada 2005 (PDF, slow download!)
Ada 2005 Tests and documentation
Annotated Ada Reference Manual (2001) -- Ada 95
Ada 83 (before Ada became an ISO language)
This is the official Ada 95 manual with annotation for compiler developers. Some link are broken since they reference documents outside the AARM. You will certainly be able to find them on the Ada IC web site and there are resources under /resources. Ada 2012 is being developed here: Ada 2012.
The following list are other links to Ada web pages:
The following is a list of documents I'm writing about the compiler being developed by Made to Order Software Corp.
The Lexical Implementation Specifications ADA Parser -- The P-Code