Gnat: The GNU Ada Compiler (2004) [pdf]
This document is a 2004 book about GNAT, the GNU Ada compiler. It covers the GNAT compilation system, its tools, and the Ada programming language from a practical perspective, serving as a reference for developers using GNAT.
Background
Ada is a highly reliable, strongly typed programming language developed by the US Department of Defense in the 1970s-80s, designed for safety-critical systems (avionics, railways, medical devices). GNAT is the GNU Ada Translator, an open-source Ada compiler that became part of the GNU Compiler Collection (GCC). Key figure: Ada Lovelace (the language is named after her). AdaCore, the company behind GNAT, was founded in 1994 by former NYU researchers who had developed the original GNAT. This 2004 book covers GNAT's architecture, implementation, and usage. Ada matters because it's still used in systems where failure can kill — the FAA requires Ada for some flight software, and it's embedded in countless defense/aviation systems. The "GNU" in GNAT means it's free software (GPL licensed).