From a related CNET article (http://news.cnet.com/8301-13578_3-20002870-38.html):

In a 2002 decision, the Colorado Supreme Court ruled that the First Amendment protects "an individual's fundamental right to purchase books anonymously, free from governmental interference." The justices tossed out a subpoena from police to the Tattered Cover Bookstore asking for information about what books a certain customer had purchased.

And in a 2007 case, federal prosecutors tried unsuccessfully to force Amazon to identify thousands of innocent customers who bought books online, but abandoned the idea after a judge rebuked them. Judge Stephen Crocker in Wisconsin ruled that "the subpoena is troubling because it permits the government to peek into the reading habits of specific individuals without their prior knowledge or permission."