What are some alternatives?
When comparing WikiSummaries and GetAbstract.com, you can also consider the following products
Blinkist - Key insights from 2,000+ bestselling nonfiction books
Four Minute Books - Making you smarter in 4 minutes or less.
Book Summaries - Great books summarized in 5 quotes (or less) so you can get the main idea fast.
12min APP - Free reading app for summaries of nonfiction books.
BookRags - Apart from book abstracts, Bookrags.
SparkNotes - SparkNotes is a resource that provided study guides for multiple different types of subjects.