Here are a few useful links to fairly detailed background reading on the NSA spying programsome of which may have been linked to elsewhere. A complete chapter of James Risen's book, which looks at what those few Justice Department officials in the know call "The Program," is now online. Two interesting tidbits: First, the NSA was given authority to determine, on its own and without any oversight whatsoever, which people they should start spying on in the first place. Second, "senior administration officials" differ on whether the intelligence has actually been used in criminal casesone of the difficulties in determining this is that, if the NSA happens to find something useful, it will "launder" the intelligence before it is distributed to the CIA or FBI to hide the origins of the info.
Also, two other scholarly links. Back in 2001, Lawrence Sloan wrote a long article on Echelon, the global surveillance system, which is pretty thorough in laying out the legal issues, and pointing out that current rules are inadequate to the task of overseeing the NSA, as technology advances and whatnot. (He believes the NSA, and Echelon, are fundamentally valuable, but just need better oversight.) On a sort of related note, Michael Froomkin has a good 1994 law article going in-depth on the problems with having an "imperial presidency," along with a follow-up here.