You can legally purchase books on how to build a still and distill liquor in the U.S. While homebrewing beer and wine is legal, distilling liquor at home is still a federal crime. Do you think these books should be banned because they instruct someone on how to commit a crime?
You can also read books on marksmanship. You could use these skills to commit murder. Should these books be outlawed too?
Because buying a book on distilling liquor could be used to start a distillery - plain and simple. The act of distilling liquor is not illegal, and that is the main focus of the book - the specific circumstances make it illegal. The same principal holds for guns. It is not illegal to own or use a firearm, and so they have books detailing how to purchase, care for, and safely handle the guns. But writing a book how to literally get away with murder is.
When you have a valid idea (i.e. making alcohol, firing a gun, having sex) and then use it for an act that isn’t intended to be part of the idea, it’s a perversion.
Greaves did not write a book about sex, that someone could then use to have sex with a minor - that would be outside the circumstances and he would be protected. However, following the purpose for which his book was written, one would have sexual relations with a minor, and if he is as knowledgeable as he claims to be, get away with it.
Now, we’ve shown the legal backing of the courts. But let’s look at the common sense aspect. If this guy really did write a book that will teach a pedophile how to have sexual relations with a minor and get away with it, then how many pedophiles aren’t brought to justice, how much emotional and physical trauma will children suffer, and how is allowing this beneficial to society. Find a similar set of circumstances where preventing this from being legal would block a legitimate right of someone.