Actually, the wikipedia entry for Arrow's Impossibility Theorem is pretty good, at least for starters.  The key thing with Arrow is not to get tripped up, like a lot of people seem to, in his use of an election game and conclude that he was just talking about a strict case of electoral politics.  

He invented an abstract voting game between three hypothetical electors to determine, mathematically, if it is even possible to conceive of a political economy in neoclassical terms, even given their pretty unrealistic assumptions, and he finds that it is not: The existence of anything like a  social welfare function -- that the well-being of a group of three or more people can be honestly conceived of as a single average or summed function -- is a false proposition.

Regarding Sen, "Freedom as Development," is the non-academic, "popular" book which summarizes his ideas pretty well, like "A Brief History of Time" was for Hawking. (The wikipedia entry for that book, unfortunately, was pretty weak, so I'd go to the source instead.)