When you perform an online CAPTCHA task (reading a distorted alphanumeric string or counting the traffic lights in a gridded photo in order to prove that you’re not a robot), is there a smile on your face? Do you feel an evanescent pride, a flush of human chauvinism, at having made short work of a request that would bewilder a machine? Don’t rest on your laurels. If you are like many internet users, you have no idea that by completing the CAPTCHA, you are helping to refine Google’s powerful image recognition technology. You have been training your own replacement all along.

This microtransaction is, as Joanne McNeil’s Lurking: How a Person Became a User demonstrates, our relationship to the internet in microcosm. The net giveth, and the net taketh away — but it seldom warns us before doing the latter, and we seldom notice until after the damage is done. It is not unlike Lurking itself, which is such a pleasurable trove of startling facts and penetrating insights that only after finishing it does one stop to whisper, “Uh-oh.”