Effectively, if a repair man arrives to fix your fridge, and happens to notice a copy of Michael Moore's book, Stupid White Men, on your counter, he would be within his remit to call the Hotline and you could, depending on how busy the local FBI field office is, find yourself receiving a visit from the federales.
Being dragged away by the FBI at 3AM for owning an un-American book is somewhat unlikely; what seems more likely is that a large number of minor things (possession of subversive literature, professing atheistic views, associating with members of environmental groups, etc.) will contribute to your suspicion rating. If you regularly make phone calls or send emails to people with high suspicion ratings, that increases your rating as well, all done automatically by data-mining software. Not to mention if your supermarket shopping patterns match the characteristics of terrorists, subversives or un-American types, in various seemingly innocuous ways. If your rating exceeds a certain threshold, additional surveillance may be conducted on you, or you may be barred from flying in commercial airliners (like those Green Party members shortly after September 11 last year). This is the Stasi for the information age; subtle and pervasive.