Sources for translations/a thought
This might get disqualified as it could be viewed as product placement and thus advertising, but it's subtle in that regard, and it's win-win.
Publishers of popular fiction with no translation into English yet would probably jump at the chance to allow sample pages of novels to be used for translation practice by Duolingo students.
There's a lot of fun popular fiction that never makes it into English.
I will also bet you that out there somewhere is an author (maybe who writes for E-books/digital/Kindle type) who would be willing to have their book totally translated by Duolingo's "Collective" method. It would make headlines. You'd need to have somebody pretty sharp watch over the finished product.
This wouldn't work, unless the writer is making the book public domain.
If the writer intends to publish/sell the book, it would not make sense to put an entire book for free. Illegal copies can easily be made, and the writer would end up losing potential sales.
What I'm thinking is finding hobbyist writers who post their stories for free (like on Deviantart), and then asking them if we could translate their stories.
Well, the sample pages idea would work. A full book would only work if it was done a page at a time and the page would be completely removed and another one is added. If done out of order, it would pretty safe except from some very determined piraters. But I agree it's unlikely.