This is a set of heavily cross linked pages which document the nitty-gritty details of each and every LilyPond class, object, and function. It is produced directly from the formatting definitions in the source code.
Almost all formatting functionality that is used internally is available directly to the user. For example, most variables that control thickness values, distances, etc., can be changed in input files. There are a huge number of formatting options, and all of them are described in this document. Each section of the Notation Reference has a See also subsection, which refers to the generated documentation.