The iPad is not a dedicated e-book reader; it obviously isn't designed to be one. But it could still take some market share from other readers, especially if the e-book producers make, or okay, iPad apps for e-book reading (as, for instance, Amazon has already said it plans to do).
What I see out of this device is hardware good for digital magazines and other periodicals, especially those with color. Magazine publishers and textbook makers have so far been frustrated by the current greyscale e-book readers, and hoping for a color device to give them the entry into the portable digital media market that they want. The iPad may be exactly what some of them are looking for.
It's been my feeling for years that creating a portable digital market for magazines and other color-enabled texts would get more Americans into digital reading than novels have so far... specialty and theme magazines, especially, are a major and so-far largely untapped market for portable digital media. Further, enabling owners to be able to "clip and save" select portions of magazines and discard the rest, would create a digital scrapbook that could be hugely popular with the public (Personal plug: I described this in my novel
Chasing the Light).
iPad looks like just the kind of hardware needed for the e-scrapbook software I described (though I can tell from descriptions that screen resolution could use some improvement).
At any rate, if people buy the iPad for reading digital magazines and similar content, or for that matter any other media, they will most likely use it to read the occasional e-book, too... especially if it is easy enough to get e-books for their device. An iPad-optimized e-book portal combined with e-book reading apps could, therefore, bring a lot of new readers to the "e" side. Those who won't buy dedicated devices will use a device they already use for other things, because... well, they have it. The same way I use my PDA for reading e-books, and other people use cellphones, a purpose for which they were not originally purchased, but which they handle admirably.
And there's every reason to expect
some owners of dedicated devices will decide the iPad is actually more in line with what they want, and will abandon the dedicated devices in favor of a multi-use, multi-media device.