Rocket Science it ain't.
I'm stealing/misquoting Julian here who's often said that bookselling isn't rocket science. I've always taken that to mean that x +y+ z doesn't necessarily give you the expected result in bookselling. If you take ten copies of book, it doesn't automatically follow that you'll sell all ten. And that's without even considering that you might want to have a copy (or lots and lots) in the window so that people can see the book. But I digress.
Last night we took along fifty copies of Mohsin Hamid's The Reluctant Fundamentalist to a launch/reading. We sold forty-seven.
I do get it wrong more often than not. So it feels nice to get it just about right.