GRASS internal: any document?

[Aime Andrea]

Hi everyone. I'm searching some document that
explains internals details about GRASS, how
does it works, something about its structure...
I know that I can "use the source", but a little
document about its architecture would be helpful
(what libraries are there, what functions they
maka available and how libraries depend on
each other). Anyone knows if such a document
is available?
Thanks
Aime Andrea