Blinkist - Key insights from 6,000+ bestselling books and podcasts
Instaread - Read or hear key takeaways of any book in minutes
Four Minute Books - Making you smarter in 4 minutes or less.
wits.io - Weekly book summaries recommended by founders and makers
Book Summaries - Great books summarized in 5 quotes (or less) so you can get the main idea fast.
BookRags - Apart from book abstracts, Bookrags.