My wife has been considering getting either a Kindle or a Nook. I've suggested she gets neither, and instead get an
Asus T91MT.
Why? You can get Kindle readers for Windows, and you can convert the T91MT to a tablet-form if you want a book-like format. The T91 is also a fully-fledged netbook with a keyboard so you can do some real work on it if you want to.
Don't bother with the standalone Kindle or Nook readers, you can get the software readers on a variety of other platforms and read the same books.