It is XXX and one of your employees is dressed incredibly immodestly. I prefer to not see so much of the body displayed when I am at the library.
We forwarded this to the relevant department head, but can offer a few general comments. The Library does not have a formal dress code, and on any given day attire varies from coat and tie to T-shirt and shorts. (Indeed, the Office staff is divided on this question, with one so-called "18th century Brit" strongly preferring neckties and their equivalent, while others ardently claiming the right to dress in the most comfortable clothes possible -- after all, who cares what one wears when answering online suggestions?)
However, department mores often include customary apparel, with those in public services adopting a stricter (if still permissive) tone. If you notice someone egregiously and repeatedly in different attire -- or lack of such -- than their colleagues, a quiet request to the chief of department would be in order.
Happily, the Chicago climate is about to render such concerns moot for another 9 months.