I'll give a nod for
Lady Chatterly's Lover.
Surprised noone's mentioned Harry Potter yet (not a nomination from me
)
I'm going to nominate
Flowers for Algernon by Daniel Keyes.
Amazon (US):
LINK
Amazon (UK):
LINK
B&N (US):
LINK
Google Play (AUS):
LINK
Looks like Canada and US have Overdrive versions for EPUB and the US also has Kindle versions.