Elizabeth Drew's "Power Grab" in the June 22 New York Review of Books is a must-have for serious critics of the Bush administration. It details, with simple rigor, various ways in which the administration has extended the power of the "unitary executive" at the expense of other branches of government.
Drew puts things most informed people know about--the nullifying "signing statement" Bush appended to the bill containing the McCain torture amendment, for example--in larger context. It turns out there have been 750 such "signing statements" during this administration and their effect has been to undermine the expressed intent of the legislature in each case. But that's just the beginning. The list of unprecedented executive encroachments goes on and on and it's all laid out, succinctly and comprehensively, in this short essay.
That comprehensive simplicity is an implicit indictment of the mainstream media, by the way. This isn't esoteric think-tank stuff. There's absolutely no reason why Time or Newsweek couldn't convey this to American citizens in a straightforward way. Maybe they will, if enough people draw it to their attention in the manageable form Drew has provided.
If you want to understand why Bush can get away with it so easily, there's a book called The Broken Branch by Thomas E. Mann that will help a lot. Mann is too much of poly-sci wonk to fully appreciate the impact of a completely mediated politics on congress, but he supplies an analysis of it's actual functioning that's enormously valuable.