[Isowen]

Well, He didn't know about it untill Gollum was captured. And when he sent nazgul to find Bilbo, he became aware of different parts of the shire. Merry, has a kind of vision of what would happen to the shire if the one ring was not destroyed, there was violence and destruction. and the he says to Pippin that there will be no shire if the war continues. So I think that Sauron is unfamilliar with the Shire, although he is aware of its presence in middle-earth, and perhaps he feels almost threatened by it because he hold little knowledge of it.