You can buy books at Amazon via your PC and then do one of two things (or mix and match):
1) After purchasing the book on your PC, you can opt to download it to your iDevice right there at time of purchase
2) Or, if you elect to send it to one device (iPad, PC, etc), you can always go into the Kindle app on any iDevice (or your computer) and, from the app's home page, you can press the "Archived Items" button (in lower left) to access any/all books you've bought from Amazon.
Only requirement is that you download the [free] Kindle app to your iDevice.
Hope this helps.