The library-related blog #UKANTHROLIB has nice post about the ethics of studying people (ethnography) in libraries.
Not surprisingly, the IRB (Institutional Review Board) shows up in the discussion. My advice is not to think of the IRB as hurdle but a friend. Their job is to make sure you get things right and don’t inadvertently harm the people you are intending to study. My experience mirrors the information in this post, in that my IRB is available review drafts of research protocols, so take advantage of that. Libraries are about people as much as we are about resources, so why not take care in the ways we study and engage our patrons.