"Cosa impari dal libro?"
Here's another problem with duolingo. My comment was about a question, I believe a multiple choice type, but because the "main point" of that question (the preposition?) was the same as the one here, using a similar sentence, my comment appears here as well wherever I made it. I've seen this happen many times, and it can be a source of confusion (as here). I don't know if I was right since we are not seeing the question I originally commented on, but I was not talking about the translation of THIS sentence.