Thank you Spurlock, that was helpful.
To expand on my previous idea: This is speculation not based on any direct evidence in the text, but I think the secret held in the Potion Book is the recipe for the potion Lily used to turn her sister beautiful. EY points out several times how Transfigurations are dangerous, points out that this is why there are still fat people etc., and Petunia herself describes the process as very long (sick for a month). So it might be possible that the potion uses Dark Magic or some other terrible secret was involved in its creation.
I'm not sure that people who actually live in the wizarding world care about appearance in quite the same way that muggles do. Wizards seem to be content to be extremely beautiful or extremely ugly.
ETA: There is now a third thread, so send new comments there.
Since the first thread has exceeded 500 comments, it seems time for a new one, with Eliezer's just-posted Chapter 33 & 34 to kick things off.
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