Trying to fact-check that claim:
from NYT: Fault has been on the Young Adult best seller list for 73 weeks.
searching their weekly overall best seller list, Fault is not currently in their top 25 nor has it made their top 25 since at least Oct 6. 2013 (thats all the farther back I have checked so far.)
that being said, this list from Publishers Weekly shows it as Amazons 14th best selling book of 2013
Book sale figures can be whack, ask the producers of Sahara
but certainly film seems to be getting much more traction than say Perks of Being a wallflower or Spectacular now did, so we shall see
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