Lars Aronsson writes

> To be clear: I need a platform where regular users, logged
> in or not, can upload new books through a web interface.
> Does that leave me with anything else than Mediawiki?

  Try http://omeka.org. I use it for teaching purposes. 

http://openlib.org/home/krichel/courses/lis654.html

  It's small enough that I can install a copy for each
  student, with a script that I run as root 

http://openlib.org/home/krichel/courses/lis654/bin/maintain_omeka

  Although primarily designed for image-based repositories,
  omeka has a bunch of plugins that you may find help
  you what you want to do. 

  Cheers,

  Thomas Krichel                    http://openlib.org/home/krichel
                                      http://authorprofile.org/pkr1
                                               skype: thomaskrichel