I sometimes wear a tie to work, and if I have to talk to the City Council or something I'll wear a suit. But I almost always wear pants and a shirt and once in a while I'll even get the shirt on top and the pants on the bottom part (I'm really proud of myself when that happens).

Seriously, the rule of thumb in public libraries is to dress one degree "up" from your public: if your patrons usually wear polo shirts and khakis, you wear a shirt and tie; if they wear jeans and a tee shirt you wear a polo shirt and khakis. Rarely do I find a library director wearing a suit or even a sports jacket as everyday wear these days, although they might have a jacket (and tie) in the office "just in case."

That's in medium sized public libraries in the US...your mileage may vary and I'm certain that folks like the Librarian of Congress wear suits to work every day. Of course, his office is somewhat different than mine.