I've suffered through many books that had a very good message, but should have been 20% as long as they were. Much repetition in every possible spot, just to end up with a book of a respectable length that people don't feel bad spending $15 on. Comprehensive summaries are a great way to get around that.
On the other hand, these summaries are (by definition) derivative works that serve almost entirely as just distillations of the book. There's some opinion rolled in, but the description of the source material goes far beyond a book review to the point that it could be seen as a time- and money-saving replacement. If I were an author, I wouldn't be happy that my book was interpreted by a random third party, let alone that many folks will assume they got the value from the work and won't buy it.