gnucash/libgnucash/engine/README

This directory contains code for the accounting engine.
Its fairly clean but far from perfect, and it certainly
lacks advanced features.

There should be no GUI code in this subdirectory, and,
ideally, it should build cleanly and independently of 
any GUI elements or assumptions.

For design documentation, please read our doxygen documentation.
This can be found at https://code.gnucash.org/docs/STABLE/
Alternatively you can build it yourself by running
 [ninja|make] doc
The result will be found in
{builddir}/libgnucash/doc/html/index.html

The same information can mostly be found directly in the header
files in this directory as well (though in marked up source format).