How is the source code of GNU Emacs organized? What are the main modules and their functionality? Where can I find an official, up-to-date description of the architecture and its source code?
Being an official GNU project it adheres closely to the GNU coding standards and directory layout. That said, if you are exploring the source tree I would start, as with most projects, with the README file in the root directory.
From that file onwards, there are several sub-directories:
`src' holds the C code for Emacs (the Emacs Lisp interpreter and its primitives, the redisplay code, and some basic editing functions). `lisp' holds the Emacs Lisp code for Emacs (almost everything else). `leim' holds the library of Emacs input methods, Lisp code and auxiliary data files required to type international characters which can't be directly produced by your keyboard. `lib-src' holds the source code for some utility programs for use by or with Emacs, like movemail and etags. `etc' holds miscellaneous architecture-independent data files Emacs uses, like the tutorial text and the Zippy, the Pinhead quote database. The contents of the `lisp', `leim', `info', `man', `lispref', and `lispintro' subdirectories are architecture-independent too. `info' holds the Info documentation tree for Emacs. `doc/emacs' holds the source code for the Emacs Manual. If you modify the manual sources, you will need the `makeinfo' program to produce an updated manual. `makeinfo' is part of the GNU Texinfo package; you need version 4.6 or later of Texinfo. `doc/lispref' holds the source code for the Emacs Lisp reference manual. `doc/lispintro' holds the source code for the Introduction to Programming in Emacs Lisp manual. `msdos' holds configuration files for compiling Emacs under MS-DOS. `nt' holds various command files and documentation files that pertain to building and running Emacs on Windows 9X/ME/NT/2000/XP. `test' holds tests for various aspects of Emacs's functionality.