If you have already grabbed 50gb from box.net then you can store your books there. It won't build library by itself but you may want to sync Calibre library with box.net using Joukuu (I have not tried it myself but planned). Only limitation will be files of size more than 25mb (and thats my problem, as the books I have scanned are >25mb in size)
If you are on iPad, you can use Bluefire reader. The manual says that it will import any book opened with Bluefire reader to its own library.