It's a book with ways to indirectly find out internet-security-ish info about things. E.g., if you're curious to know whether visitors to your website also frequent the San Francisco SPCA website, you might try displaying an image from that website and time how long it takes for that image to appear. If a visitor sees the image quickly, their computer probably already had info from the SPCA website cached on their computer from previous visits.
Some of these techniques are practical. Some others, uhm, sound like some grad students dared each other find the most bass-ackwards way to "leak" information from systems they already owned. E.g., whoever figured out how to guess at network traffic by using a high-speed camera to monitor lights on a modem, uhm… these people were clever but maybe could have found a better use of their time?
Practical or not, this is some fun reading if you're into that sort of thing.