I find most of my reading material on the Usenet. Places like forum.nox.to, ebook-land.cc or even ibooks.to are the backup sites I turn to when I can't locate something. Yeah, downloading from OCH may not be very convenient, but for small ebook files it's acceptable. If there's a torrent tracker like MyAnonamouse for German language ebooks I don't know of it, whereas the online libraries that have been mentioned (Anna's Place, Zlib)j only offer a small fragment.
yes, mam has some books in german but mainly older stuff and 99% from ibooks. i used to offer but got tired of their inconsistent rules/category system and being forced to provide bookcovers. there was/is also not much interest and even less appreciation for uploaders in general.
ipt has sometimes french, spanish stuff (dunno if still there ) but i have never seen german.