I've just listened to a marketing guy who is selling people a program on how to make money by writing a summary of a successful book and publishing it as an ebook.
His advice is to pick books that are longer than three hundred pages, for which the publisher sets the price. Also not to use too many actual excerpts to avoid violating copyright, and keep your summary under 50 pages.
Is it legal? Seems to be. Is it ethical? Hmmm...yes, maybe reading a summary might drive a few people to buy the full book, but probably it will cannabilize sales more than it increases it.Frankly, I'd feel sleazy using this tactic.
In fact, I wonder how the marketing guy would feel if I bought his course and then published my summary of it for $49 instead of his price of $97?
My suggestion: if you think your book might be a target, beat them to the punch. Create your own digest version; if people have the chance to buy a summary from the original author or from a stranger, they'll go for the author every time.
(For more respectable approaches to creating and marketing your work, see my book, "Your Writing Coach," published by Nicholas Brealey and available from Amazon and other book sellers.)