Building Recommender for book paragraphs
You would have to come up with a metadata set of each paragraph and then create a content-based recommendation system based on that metadata. You could also do a community-based recommendation system if you have extensive user data. 

As for the metadata set, you could use something like n-grams to "summarize" each paragraph and then have an index for each. From there, you would know what people like and then algorithmically choose the n-grams that person is most interested in and find the paragraphs that correspond to those n-grams.