Calisto isn't bad, but Warnock has a little better horizontal density without sacrificing any legibility.
Honestly, there are a zillion fonts, and you could spend an enormous amount of time wading through them without gaining much. At the size they're being rendered on a Kindle, most of the differences are very subtle—mostly you'll notice they have different aspect ratios and x-heights.
Generally speaking, a higher ratio of x-height to ascender height yields more readable bulk text, so that's what I look for. I also stick with only Pro fonts because they tend to have better hinting and kerning, and wider character sets.