Never wear a suit for a library job interview. Instead wear a threadbare blazer, unpressed khakis, a stained tie, and well worn shoes. Women... if you can't stomach an ill-fitting pants-suit, wear a dowdy skirt and an oversized cardigan. Remember, most search committees are suspicious of style.
Plus, if you dress too well, it becomes obvious to the potential employer that you have unrealistic expectations about library salaries and their ability to keep you dressed in your fancy clothes.