You should look into Calibre, it's library management software for e-readers, and it works wonderfully with a kindle.

You can convert between lots of different formats and load them to your reader from your PC or Mac.

I've loaded books from Google's service and public domain stuff from Project Gutenberg and archive.org. I've loaded some PDFs on it which are kind of janky, but sometimes workable depending on the book.

But basically, I'm not worried about being able to read a book on my kindle unless it's a PDF.