It may have been overlooked during all the exciting announcements of WWDC this week, but the latest version of iTunes has incorporated the iBookstore into iTunes, so you can browse and buy eBooks from your computer, not just your iOS device.
Unfortunately, eBooks purchased from the iBookstore can still only be viewed on iOS devices, so forget about using the iBookstore to shop around for the best price for your Kindle or Kobo reader, unless you’re prepared to start stripping out DRM.