Download Calibre for Mac. When you plug the device in, the ebook library manager recognizes it, and you can directly transfer your books from within Calibre. Calibre will also convert books from many formats to .mobi. If you primarily download from feedbooks.com, just download them in that format. If you intend to read them also on your iPhone, convert also to ePub. Calibre is available from
http://calibre.kovidgoyal.net/.