Greg Conti -- a West Point instructor in computer science and information war -- has taken a long, hard look at the amount of information Internet users explicitly and implicitly disclose to Google and the results, collected in his book Googling Security: How Much Does Google Know About You? are sobering.
Conti enumerates all of Google's (often fantastic) services, describes how compelling they are, and then notes what information you disclose when you use them -- even when you only use them inadvertently (say, when you send email to someone with a Gmail account, or when you load a bookmarked Gmap that's been sent to a group of logged-in Google users, thus tying yourself to those users as part of the same group).
In slow, methodical steps, Conti builds his case: our complacency, Google's capacity for building compelling services, and the inadequacy of our browsers and other tools in alerting us to potential information disclosure have created a situation where Google ends up in possession of an alarming amount of information about us, our beliefs, our movements, our finances, our health, our employment and our social circles.
Conti's explanations are extremely accessible, even when discussing difficult and counter-intuitive subjects like cross-site scripting and cookies. Likewise accessible are his concrete recommendations for staunching the flow of personal information from your computer into Google's records. Finally, Conti does a great job of explaining why people who "have nothing to hide" might still want to keep their information to themselves (the approximate dimensions and characteristics of the body under your clothes aren't a secret -- but you still don't walk around naked in public and you'd resent it if someone forced you to. Private and secret aren't the same thing).
I've given the subject of privacy and Internet use a lot of thought, but even so, Conti's book opened my eyes to potential risks I'd never considered. I'd recommend this to anyone who's worried about what's happening to our ability to control the aggregation of our personal data.