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What could Amazon do to increase its growth rate? Generally speaking, step-changing seeking companies can follow three approaches:

Dramatically lower prices to appeal to cost-constrained consumers. Following low-cost strategies can help to ward off emerging attackers and grow a market. A Kindle that sold for less than $200 would expand the category, as long as the device didn't trade off too much functionality to reach that price point.
Provide additional benefits that appeal to new customers. One of the Kindle's benefits is its ability to connect seamlessly to a wireless network. Amazon could create a location-aware Kindle that provided real-time customized guide books. This approach could even let Amazon branch into new business models, such as location-based advertising. Of course, companies have to be wary of creating Swiss Army knives that cram on additional not-very-good features, but adding the right set of additional benefits can expand consumption.
Bring a product to new circumstances. For years, Pfizer's Listerine mouthwash was a mainstay in many bathroom cabinets. A few years ago the company (whose consumer brands are now owned by Johnson & Johnson) introduced portable "PocketPacks" that allowed consumers to get Listerine-like benefits on the go. The Kindle's underlying mobility makes a circumstance-based play a bit of a stretch, but perhaps there are opportunities to build Kindle-like functionality into other devices so that people could enjoy their content wherever they happen to be.
One could argue that the ability to seamlessly translate text to speech fits in the second category, though it's hard to see pilfering some of the audio book market as a true breakthrough move for Amazon.
Amazon still seems to have a success on its hands with the Kindle. If it keeps incrementally improving its device, it will continue to build a nice business. If it has another disruptive trick up its sleeve ... watch out!