I prefer kerning, but not embedding the font.
Do not alter the book text adding specific spacing that will look nice in this or that font on this or that reader, but rather rely on the device's font rendering to do the right thing. Sure most devices will have a crappy font with no proper kerning, but that's not the book's fault. Personally, I edit my preferred font to fix kernings and upload it to my device, so I can use it for every book (without embedding).
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