Just wanted to post to add to the Calibre love
I have a Sony 300 and all of my books are in epub. I am awaiting the K3 and I used Calibre to convert them all to mobi. So right now I have a each book in drm free epub and mobi format. When my K3 arrives I will plug it in and xfer all the books via Calibre. It knows to transfer all the mobi files and not epub. It really is that simple (or so I'm told
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When I converted, I switched the output to Kindle, and did it in batches of 20 at a time. I also went into "Look & feel" and checked "Remove spacing between paragraphs" and that will also add a 1.5em indent to the paragraphs. You can also check the box below it to "Insert blank line" between paragraphs. It depends on if you like spaces between the paragraphs or if you want to use as much real estate as possible and cram as many words on a page as you can. Lastly, under page margins I set the 4 margins to 1.
Now this is my preference, ymmv and you need to play around with the settings to find what looks best for you. Just take one book and filter it through a few times and look at the output.
Enjoy!