It was much remarked recently when the Economist published an article commenting that "so many business books are awful."
One problem is that most books are given positive reviews.
I think this is something that probably has very little to do with any book itself. When reading, the reader takes time to sit down and think. They get a chance to let their mind wander on a subject and find a new angle.
I'm hardly an expert on business, and it's certainly true that I could use more formal business education. But most business books seem very good to me at the time, but the lessons I've "learned" are as longlasting as lightning in the sky.
So I've come to view business books as good for inspiration and new perspective. Sometimes that's worth my $20.
(inspired by John Moore's review of "The Pirate Inside," which I confess does look interesting).