I also use http://gen.lib.rus.ec/ to get all sorts of ebooks that I process with the Calibre app to AZW3 + fix metadata so it works with Kindle+Goodreads.
Is there a particular reason why you chose AZW3 instead of KFX? I'm no expert but it was my impression that KFX is more modern and has better options on the Kindle.
It seems to be required for goodreads integration so I can keep track of what I've read.