What are some alternatives?
When comparing WikiSummaries and Four Minute Books, you can also consider the following products
Blinkist - Key insights from 2,000+ bestselling nonfiction books
Book Summaries - Great books summarized in 5 quotes (or less) so you can get the main idea fast.
Instaread - Read or hear key takeaways of any book in minutes
GetAbstract.com - Book Summaries, Book Abstracts
12min APP - Free reading app for summaries of nonfiction books.
BookRags - Apart from book abstracts, Bookrags.