Nobody has ever captured the nature of power on an individual level to the depth and breadth that Caro did on this book. (except perhaps his epic treatment of Lyndon Johnson)
Over something like 1,100 pages you get to track the career of an aspiring reformer as he transitions to skilled and trusted government official, to someone who manages to grow to the point that he is more powerful than the Governor and Mayor of New York during NY's economic peak -- despite never having been elected to anything. Then you get to witness his decline and ultimate fall.
This is probably the best biography ever written. It may take you six months to read, but its time well spent.
Lately I've been reading Foucault and I find that many pieces of The Power Broker are incredible examples of Foucault's post-modern/post-structuralist theory of power: power relations as a sort-of amorphous "lines of force" that move between people through society, occasionally emergent as structural domination/power, rather than as some sort of antagonistic relationship between rulers and ruled. This conception of power makes sense when you consider Moses operating at an intersection between (and attempting to leverage) many different "fields" of powers: government politicians, wealthy private estates, union high-ups, the news media, etc.