Amartya Sen won the Nobel Prize in Economics for his work on the philosophy of economics. My own intellectual life was greatly influenced by the good fortune of attending a lecture Sen gave to my undergraduate department my sophomore year, and I spent the next few years working my way through as much of his writing as I could. Much of it was way over my head, but a few books really helped me understand his concepts. Of these, I personally considered Commodities and Capabilities to be the one that did the most to make Senist philosophy comprehensible to me, and it would be the starting point that I would point a student to if I were trying to introduce someone to Sen (this is not a hypothetical; I just ordered a replacement copy specifically for this purpose).
If you cannot get Commodities and Capabilities, then you could also go with The Standard of Living as a decent introduction, especially if you want to approach his ideas from the standpoint of utility theory. Sen pulls everything together in his masterwork, Ideas of Justice, but I wouldn't want to read that without an introductory book like Commodities and Capabilities.
So I strongly recommend this book as a good point to start reading Sen. But what if you don't know yet whether you want to read Sen?
I have tried many times throughout the years to give a short summary of Senist thought, and it is not easy. I am going to offer four basic points:
1) Sen is heavily influenced by Rawls, and much of his writing can be seen as a reaction to Rawls. (In his comments on a collection of essays on the capabilities approach, Measuring Justice, Sen noted that he felt as though he were being lowered into a pit to fight Rawls, and that if he had to do so, he insisted on wearing a Rawls t-shirt into the fight.)
2) One of Sen's main points of departure from Rawls is in looking for a theory of justice that allows you to compare different states of the world and evaluate incremental change, instead of focusing on a world of ideal justice.
3) Sen deeply respects differences across individuals, especially regarding their preferences and needs. This leads him to react strongly against measuring well-being in terms of the actual goods you have. For example, he often discusses the very different justice implications of somebody starving because he owns no food v. somebody starving because he is choosing to undergo a religious fast. (This is hardly a new idea, of course. In the middle of the lecture I saw, Sen decided he wanted to quote Adam Smith on this point, and asked someone to find the relevant passage by looking up the term "linen shirt" in the index to the Wealth of Nations. Do it.)
4) This leads Sen to perhaps his most famous point, and the point that his book focuses on: that we should evaluate human well-being not in terms of what people have but in terms of their capabilities, of "what they can be and do." Capabilities might include things like "walking down the street without shame."
Sen aggressively avoids suggesting a definitive list of key capabilities, which is probably wise of him; I think that he wants to focus on the bigger picture of what a theory of justice should look like, without getting into endless fights over specifics of implementation. Nonetheless, I think it helps a great deal to understand Sen if you look at Martha Nussbaum's more concrete application of his work, either Women and Human Development (which is quite relevant to men as well!) or Creating Capabilities.
If you're only going to read one book of Sen's, it should be The Idea of Justice. But you could probably read both Commodities and Capabilities and The Idea of Justice more quickly than you could read just The Idea of Justice by itself.