I see someone asked above about the differences between the LCD and eInk versions of Bookerly. Since I got access to both I took a look. There are some very minor changes in the base contour of the glyphs:
but the main change is in the hinting used, with the eInk version hinted to render much bolder:
Speaking about hinting, I tried adding malformed hints to Fontin which would distort the characters. On ADE you can just see the effects of the bad hinting, but I couldn't see any change in rendering on the Kindle, meaning that (as we should expect by now) the Kindle doesn't use hinting on embedded fonts (example pics attached below).
On a 300dpi device this isn't a big deal. Anyone who's bought an ePub where the publisher's lazily embedded Charis to cover all the glyphs will know that bad hinting is worse than no hinting -- Charis is notorious for its badly-speced alignment zones that cause cobblestoning.
[Edit]After posting this I realised I'd been using a font with CFF outlines and Type1 hinting for the test of embedded font hinting, so went back and manipulated a different font that used TrueType (quadratic) outlines and TT hinting (which is completely different). This
did show a difference, indicating that the hinting engine is not actually turned off for embedded fonts, but only works for those with TT contours. In retrospect, given the differences shown above, I should have tried this initially.