If Amazon.com ever wants to make the Kindle truly ubiquitous, it will have to find some other sales channels beyond its own website. At least, that's the upshot of a new report on the electronic reader market from Forrester Research. The report says the first wave of tech-savvy e-reader adopters were an "easy sell" for Amazon. But the online retailer "isn't well positioned" to keep control of the market as it matures, opening the door to rivals like Sony, Borders, and even Wal-Mart to grab the next wave of e-reader consumers, the report concludes.

Here's how Forrester analyst Sarah Rotman Epps describes her conclusions in a blog post:

While early adopters of eReaders were a perfect storm of demographics for Amazon (they could afford the device, they have a need for the device in business travel and urban commuting, they like technology, and they buy lots of books online), future prospects for the devices look completely different. They're more likely to be female, less tech optimistic, and they read a lot (on average, 5 books per month) but they buy and borrow books from multiple sources, as opposed to buying lots of books online.

The big takeaway is that this could spell trouble for Amazon, if competitors can move in to better serve the later waves of adopters who don't have as strong a relationship with the eCommerce giant.

The Forrester report estimates the number of e-readers sold in the U.S. will top 3 million by the end of this year, and hit 13 million by the end of 2013.

Could Amazon ever team up with a brick-and-mortar retailer to sell Kindles, or even establish its own outlets?

Amazon has set up a website connecting Kindle enthusiasts with people thinking about buying a Kindle, and it's developed a Kindle app for iPhone users, but so far its marketing and distribution efforts have remained online.

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