24symbols, as recently gone into beta on a book publishing service that offers both ad-supported and subscription options. They are looking at reading e-books as a web activity, perhaps one that you do along with friends. They're exploring a variety of models, including the freemium model. It's a way to offer content in a variety of payment plans, offering a way to find more readers for content.
This kind of experimentation is important as we all are finding out what works and what doesn't. I'm sure the nature of e-books will continue to evolve rapidly. Which means that something that's the right course of action today may not be correct tomorrow. Stay flexible, and stay in touch with what's happening out there. Your profits may depend on it.