Insight Density is the number of Aha moments for every 100 words. A good example is the series Two minutes with Seth Godin that Blinkist ran. Each 2-minute episode in this 50-episode series forced one to think.
One can employ Artificial Intelligence to detect which sentences are insight-dense. For example, TextRank finds how similar each sentence is to all other sentences in the text. The most important sentence is the one that is most similar to all the others, with this in mind the similarity function should be oriented to the semantic of the sentence, cosine similarity based on a bag of words approach can work well and BM25/BM25+ work really nicely for TextRank.