www.feedbooks.com is my favourite repository, and I use the Mobipocket format. It's nice because the recommendation links take you to more stories and they download automatically. I'd recommend you start with searching for author H. Beam Piper, and under-appreciated author. My favourites from him are Space Viking, Little Fuzzy, and Rebel Raider. All books on Feedbooks are free, and there's also a substantial number of non-English books. Try searching for Dumas or Verne if you're wanting to practice your French, for instance.

My second favorite source is the Baen Free Library (google for it). Books have to be individually downloaded and imported into FBReader, but there are books from top-flight sci-fi authors. Again, free. Start with David Weber's book "On Basilick Station".