Ok.. after a few days of a quilt book and a craft subscription on both the Nook and the Kindle, I can tell you that diagrams are very difficult to see on either the Nook or the Kindle. The screen is just too small and you can't enlarge it. However, you CAN use the free Kindle or Nook apps on your computer and they look great. So, unless you have your computer right by your sewing machine (and I know some of you do), I can't see ever buying a Nook or Kindle version of a quilt book. I do think an Ipad would have a large enough screen to see detail.
I bought the book Crazy Shortcut Quilts by Marguerita McManus. This works fine on the Nook (or Kindle) as the directions are easy to read and the diagrams are still readable even on a Simple Touch. Some of the detail for crazy quilting is lost and I had to look for that on the enlarged version on the computer. The craft subscription, Cardmaker, has the same problem. You can read the text just fine, but the detail in the photo isn't all that easy to see unless you are looking on your computer. Doable, not comfortable.