Amazon has debuted the Kindle Fire HDX, a tablet with a 7- or 8.9-inch high-resolution screen and a host of upgraded features, including a special mode that keeps the gadget going for 17 uninterrupted hours of reading. The 7-inch version starts at $229, with the Wi-Fi-only model available in mid-October. The 8.9-inch version starts at $379; its Wi-Fi-only model will be released in early November.

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