There are people who read the occasional book--the new Stephan King or Harry Potter--and then there are people who Read books. My wife and I each got a Kindle about a year ago. Looking at the price savings on books, as well as bookcases to store them, a Kindle was a smart choice. I read a lot and over the past year have 35 books finished on the Kindle. My wife on the other hand is in another league--fast reader + insomnia = she has 201 books read! The savings has more than paid for buying the Kindles.
For people like us, the Kindle will probably win over the iPad for reading. For a decent percent of people like us, I'd bet they've already invested in a Kindle too. But we're a small minority--for everybody else, the ability of the iPad to do everything else too wins.
The big loser are bookstores. Barnes and Noble used to be a regular visit to get more books--not so much anymore.
For every category of device, there are the hardcore users, and then there is everybody else.
Photographers will have a great expensive camera with good lenses, but most of us will settle for the cell phone camera because it's convenient. Some people spend a ton on a great home theatre, many buy a TV at Walmart or Costco. Avid readers will choose a Kindle or other dedicated one, most will use an iPad or their phone.
There are always a lot more people in the second category--that's who Apple is targeting with the iPad.