The Anarchist Cookbook (1971) is infamous in some circles because it is filled with recipes for drugs and explosives; it is infamous in other circles because so many of the recipes are rubbish. But what, you might wonder, did the Federal Bureau of Investigation think about it?
Apparently it thought the cookbook "has to be one of the crudest, low-brow, paranoiac writing efforts ever attempted." The quote comes from a memo in the FBI's files on the book, which have been released and posted online [pdf]. The cache also includes worried letters from concerned Americans ("Mr. Hoover, this is not a cookbook!"), replies signed by J. Edgar Hoover ("With respect to your question, the FBI has no control over material published through the mass media"), summaries of the book's contents, inquiries into whether the book violates the law, reports on which stores stock the book, and more. There is also some discussion of one of the unrelated online Anarchist Cookbooks, which the FBI acquired on a floppy disc; the files duly include photocopies of the disc. All in all, a collection far more interesting than the book itself.