This page tells you how to load books via the pc
http://www.amazon.co.uk/gp/help/cust...deId=201506260
If you buy lots of books from other sites it might be time you looked into loadig them all into Calibre so you have a library on your pc. Calibre will recognise your kindles and you can use it to send books via the usb lead, but you need to load your books into Calibre first.
http://calibre-ebook.com/