Well, that depends. First, there's a big difference between an Amazon best-seller in a single category and a New York Times best-seller. The former you can achieve with a coordinated effort of your existing network. The latter requires investment in comprehensive marketing and publicity campaigns.
For most authors, books serve a particular business purpose: access to higher-profile clients, commanding higher fees for speaking and consulting engagements, add-on sales for contracts, or even a high-quality business card for networking. Let's face it, most of us won't retire on our royalty checks. But if we're smart about how we position ourselves and our books, we can create additional opportunities and income streams.