"A Philosopher's Calling," by Ruth Barcan Marcus.
Brian Leither links to Marcus's 2010 Dewey Lecture, which she approached as an intellectual autobiography. In addition to briefly describing some of her work, she gives some sense of her personal history and what it was like to be a woman in the philosophy department: "Yale had a philosophy club open to undergraduate and graduate students. I was elected president but then received a letter from the chair of the department suggesting that I decline. The reasons given were that Yale was predominantly and historically a male institution and that my election may have been a courtesy. Also, the club's executive committee met at Mory’s which was closed to women. I did not respond to the letter and did not decline. It was, to me, obviously unreasonable."
"Actualism," Stanford Encyclopedia of Philosophy.
Marcus is best known for her Barcan Formula (Barcan being her maiden name), which holds that if something is possible, it actually exists. (Or as she put it in the Dewey lecture: "There is, on my account, no inflated metaphysics of possible worlds, except as a façon de parler. Possibility is about the way the actual world might be.") The Stanford Encyclopedia of Philosophy gives a primer on actualism and possibilism,
"Whose Idea Is It, Anyway?: A Philosopher's Feud" by Jim Holt in Lingua Franca (via Graeme, @gcaw).
In 1994 philosopher Quentin Smith argued that Saul Kripke's New Theory of Reference actually draws heavily on Marcus's early work and should be credited accordingly. Within the academy, this was considered an explosive and borderline libelous accusation. Holt's article is an amusing exposition of a philosophical controversy and--despite the fact that the central dispute concerns the origin of several fairly esoteric ideas--it helps put Marcus's work in context with that of Kripke and other logicians.
"Moral Dilemmas and Consistency" (PDF) by Ruth Marcus, the Journal of Philosophy.
In later years Marcus focused on moral philosophy (it's not a huge departure from her previous work if you consider that a lot of moral questions have to do with how things are and how things might be). This paper about moral dilemmas is thoughtful and tolerant. She argues that moral dilemmas are real and difficult, and we shouldn't try to wriggle out of them with sophistry: "For dilemmas, when they occur, are data of a kind. They are to be taken into account in the future conduct of our lives." She also advises an ecumenical approach with regard to their resolution: "Not all questions of value are moral questions, and it may be that not all moral dilemmas are resolvable by principles for which moral justification can be given."