Justice (Farrar, Straus, and Giroux 2009) by Political Philosopher Michael Sandel is based on his famous undergraduate course of the same title at Harvard. The book is, in essence, an introduction to basic issues in and approaches to justice, aimed at an undergraduate (or otherwise inexpert) audience. It includes discussions of utilitarianism, libertarianism, Aristotelian and Kantian ethics, often in the context of interesting ethical dilemmas. I plan to give the book to my daughter, just as soon as I've finished with it. So far, my sense is that Sandel underrates consequentialist theories (including utilitarianism) and overrates the practical utility of motive-based theories such as Kant's. It is a delightful read nonetheless.
The Idea of Justice (Harvard/Belknap 2009) by Nobel Laureate ('98) Amartya Sen, is a far more ambitious work, which rejects ideal theories of justice, which he finds ultimately untenable, in favor of more workable ideas of comparative justice (and injustice). Recognizing that perfect justice is probably unattainable, Sen is concerned with how public reason can be exercised to reduce injustice. Moreover, as an economist, Sen appreciates that practical ethics often requires choices and trade-offs. Although not as simple in its approach or presentation as Sandel's book, Sen's book is, like all his works, clear and eminently readable. It expects more of, and gives more to, the reader.
For a simple proxy by which to distinguish the relative seriousness of these two books, just look at the blurbs on their back covers. Sandel's book has recommendations from two prominent and very intelligent newspaper columnists, E.J. Dionne, Jr. and George F. Will. Sen's book has recommendations from the likes of Harvard Philosopher Hilary Putnam, the late Oxford Political Philosopher G.A. Cohen, and fellow Nobel Laureate Kenneth Arrow.