The Sixth Floor Museum at Dealey Plaza has become a serious international repository of documents, films, periodicals and research dealing with the Kennedy assassination, much of which the museum is now making available to the public in a sunny reading room staffed by a full-time academic research librarian. You do have to call ahead for an appointment, but the reading room is designed to be open and welcoming to all interested persons, according to Nicola Longford, executive director of the Sixth Floor Museum. The full collection amounts to more than 35,000 items, all of it indexed with the latest software and accessible through touch-screen monitors at small work stations. But instead of carrels in musty stacks, these work stations are desks next to big open windows looking out on the site of the event. Opened June 29, the reading room is an exciting addition to the life of the city.