This is akin to say that you'd feel like you overpaid (or wouldn't hire) a programmer, given that he wrote a book on a subject that you could study and then use

This is akin to say that you'd feel like you overpaid (or wouldn't hire) a programmer, given that he wrote a book on a subject that you could study and then use to implement the code. A book can contain actionable advice, but it's still somewhat limited and generic. The value of a consultant lies in his ability to apply his knowledge to your specific requirements. It's a personalized service that no book can replace. In fact, writing a book usually leads to more consultancy gigs and higher rates.