Use Calibre to convert your ebooks into .mobi format.
Open iTunes with your iPad plugged in, click on the device, then apps, then scroll down to the File Sharing section and click on Kindle. The list of documents will light up.
Open the directory where your .mobi files are kept. Click and drag them to the Documents pane to transfer them.
AFAIK there's no automatic way to send from Calibre to Kindle. I may be wrong, though.
|