I'm searching for an extensive description of the lrf structure (something like the pdf reference only for lrf's) ... does somebody have an address?

There isn't such a document (publically available, that is). There is some basic description at LibrieWiki:
http://www.sven.de/librie/Librie/LrfFormat
Beyond that, the best source of info is probably lrf2lrs from Calibre sources.

There is a description on www.y-adagio.com/public/committees/iec_pt62448/1_np(0509)/100agm368.pdf
It is a document dated 2005, so I do not know if there have been any changes made.

Wim