Another option is Summarist.ai, which uses the GPT-3 language model to create readable summaries of books. The service offers more than 1 million summaries across categories, and can create a summary in less than 30 seconds. It's good for educational, informative or personal use, as long as you're not violating copyright.
If you want to expand beyond books, Unsummary will summarize books, movies, TV shows, podcasts and more. It's geared for researchers, students and anyone who needs to quickly get up to speed on new information. It can summarize material in a few seconds, making it a good way to stay current.