There's no need to look at the library listing if all you're using it for is to buy books and transfer them to Calibre for conversion.
All I do is:
1. Buy the book from the Amazon web site and tell it to send it to "Kindle for PC".
2. Run K4PC and let it download the book.
3. Open Windows Explorer, navigate to "My Documents\My Kindle Content" and sort the display by descending date order.
4. Drag and drop the new book(s) from the Windows Explorer window onto Calibre.