The Kindle DX has a larger screen than its predecessor yet still cannot portray more than 16 shades of gray.

AP / Mark Lennihan

Lots of novels have the potential for one good sequel, but not too many can sustain a story over a third book.

Amazon’s new Kindle DX is like that extra sequel. This $489 wireless tablet doesn’t do much to advance the electronic book narrative beyond Amazon’s earlier Kindles. It does, however, introduce two plot elements that might play a larger role in future e-book chapters. The catch is, neither of those changes involves the Kindle DX’s most obvious upgrades.

The big one is its screen: Instead of the $359 Kindle 2’s 6-inch display, the DX includes a higher-resolution, 9.7-inch version that approximates the dimensions of a hardcover book.

The DX uses the same electronic-ink technology as its little brother, so it offers similar benefits and frustrations. Text appears sharp and flicker-free, even in sunlight. But the DX’s screen still can’t match the contrast of newsprint, still takes a second or so to draw a new page (after first flashing a photo-negative image of the current page) and still cannot portray more than 16 shades of gray.

The DX can also rotate its onscreen image to match how you hold the thing: If you grasp the DX sideways, the display automatically pivots from its standard portrait orientation to a widescreen, landscape mode. The DX will even invert the image if it thinks you’re holding it upside down, though that’s more distracting than useful.

The DX’s larger display contributes to its slightly heftier weight, just under 19 ounces. Amazon’s $49.99 leather cover adds almost 11 ounces.

It does not, however, seem to affect the DX’s battery life, which Amazon estimates as “up to 4 days with wireless on” — the same number it cites for the Kindle 2.

With the bigger display comes more storage, but that doesn’t yield a major improvement either. Amazon says that the DX’s 3.3 gigabytes of flash memory can hold 3,500 books, up from the 1,500 you could squirrel away on a Kindle 2 — but how many books do most people need to carry at once anyway?

Those upgrades also can’t do much to improve the already-excellent experience of buying books using the Kindle’s Sprint-provided wireless connection. To its credit, the Seattle retailer’s selection keeps growing: As of late last month, the Kindle Store carried 307,420 books (note that some cost as much as print editions sold on Amazon), 40 newspapers, 30 magazines and 5,775 blogs.

The DX’s more useful contributions come in its software and marketing.

Unlike older Kindles, the DX can display PDF (portable document format) files without prior conversion — plug the device into a computer with its USB cable, drag and drop PDFs into its “documents” folder, and those files will appear on the DX’s home screen.

Like other PDF viewers, the DX doesn’t magically turn a PDF into text that you can search, view in your choice of type sizes or have read aloud by the Kindle’s text-to-speech software. But because so many documents circulate in this format — from scientific papers to public-domain e-book downloads to files you may have created on your own computer — this still constitutes a big step forward for the Kindle.

When future e-ink screens can display more shades of gray or even color, making maps and pictures easily readable, the Kindle’s newfound PDF fluency will matter even more.

With the DX, Amazon is also exploring cheaper ways to bring this device to readers. Three newspapers — The Washington Post, The New York Times and the Boston Globe — will offer the DX at a discount to readers who subscribe to their Kindle editions. And five schools (Arizona State University, Case Western Reserve University, Princeton University, Reed College and the University of Virginia’s Darden School of Business) will provide DXes to students to use as textbook readers.

Those deals get around the relatively high cost of a Kindle, but few readers can benefit from them. The newspaper discounts, for example, only cover readers living outside of each paper’s circulation area — an arbitrary restriction that will probably prove to be self-defeating.

Even with a steep discount, any Kindle suffers from Amazon’s “digital rights management” usage controls, which do not allow you to loan or resell a book. Amazon’s failure to deliver Kindle reader software for any other device but Apple’s iPhone and iPod touch further limit the utility of a Kindle purchase.

Paper books don’t have those problems. Nor do the DRM-free MP3 music files Amazon sells on its own site. The next draft of the e-book concept — whether it’s from Amazon or somebody else — should draw from those examples.