The Rationale for Ada 2005 provides an overview of Ada 2005 features, examples of their use, compatibility with Ada 95, and more. It was written by John Barnes, and was sponsored in part by the Ada Resource Association. This is an unofficial description of the language; refer to the consolidated RM for detailed language rules.
The Rationale is available in HTML for on-line browsing and PDF for printing. Note that the HTML version is slightly different than the PDF, as it includes an an enhanced Index, a Search function, and of course hotlinks.