The fedora on the covers, if I remember right, started out as a visual short cut to say to perspective readers "Wizard+Detective". Plus, the art department just thought it looked better. Once the cover artist read the books (a few years after they started up) he painted covers both with and without hats and they always thought the fedora looked better. Eventually, it became a kind of beloved inside joke between author, publisher and fans. I'd be shocked if we ever got a cover without the fedora now.