I am by no means expert on Sen, but I have read one of his papers (in the context of a college course on political philosophy), which you might find useful. Try to look up his paper, "Equality of What?". This addresses a twist of your question. Namely, the same challenge that confront those interested in development, also confronts those interested in evaluating distributive justice in a society. In particular, if you believe in a (partially) "egalitarian" conception of distributive justice, you must specify the metric to evaluate the degree to which a particular society or outcome is egalitarian. What I take to be a major point of Sen (and I could be totally wrong, so take this with a grain of salt), is that it is *far* from obvious how to specify such a metric. In the paper I allude to, he contrasts his "capabilities-based" notions with more common notions in political philosophy. (For example, utilitarianism -- which in the context of development, one might compare to the use of a naive wealth statistic like GDP as a decision-making criterion. As another example, he also considers Rawls' "minimax" criterion, from "A theory of justice".) It has been a while since I looked at the paper, but I remember sharing some of your skepticism regarding how *useful* the capabilities-based framework is. I consider it an open question whether *any* way of "operationalizing" it (to use Tyler's term) would in fact reduce it to one of the alternative metrics Sen criticizes. But again, I don't know enough about Sen's work to have a lot of confidence in my view. It might be helpful to consult the Stanford Encyclopedia of Philosophy (http://plato.stanford.edu/entries/capability-approach/).

Development as freedom is very good, at least the first three chapters (if I remember correctly). It is not a casual airplane read, but it really showcases Sen as a first rate philosopher, someone that can think in a novel way and explain that thought intelligibly

Identity and Violence was also pretty good.