What is http://launchpadlibrarian.net (besides "a file repository used by Launchpad")?
What is there? How do you find out where things are? Are the urls permanent?
In particular, does it contain ppa .deb archives? Or is there some other way to get .debs for ppa packages?
The librarian is simply a file store. From it's documentation in its source code:
Decoupling file storage from the rest of Launchpad presumably makes it easier to scale out. As far as I know, the urls are permanent, but it is not designed to be browsed by humans.
While there was talk of using the librarian for PPAs, I don't know if that has happened. You can see that the actual archives that contain the PPA packages can be found at:
http://ppa.launchpad.net/$(launchpad_user)/$(ppa_name)/ubuntu/
For instance,
ppa:andrewsomething/typecatcher
is at:http://ppa.launchpad.net/andrewsomething/typecatcher/ubuntu/