I have started reading a novel in Luxembourgish on my e-book reader ("Manu" by Gast Groeber, also available on iTunes).
Amazon's Kindle Store has an "Other Languages" section with a lot of interesting, albeit often mislabeled stuff: You can find books in Icelandic (in the "Icelandic" section), Slovene (under "Basque"), Russian (spread among several sections such as "Alsatian" and "Corsican"), Scots, Scots Gaelic and many more.
If you have an e-book reader or e-book software and some money to spare, it's worth browsing through the "Other Languages" sections. They might not have what you're looking for, and they might not have it in the right section, but it's a treasure chest for polyglots.
And of course, other e-book stores or web sites may have many other books in less mainstream languages...