The best books on Satanism

Trouble is that most things labelled "Satanist" are not. Even The Satanic Bible is a libertarian tract at heart more than anything. It's little more than a tag for everyone with a grudge against society to hang their hats on rather than an organised set of beliefs. Also, being Christian in it's very nature a lot is just reverse Christianity such as the Black Mass and other nonsense. the only thing I have ever read that comes close to what people would think of as "Satanic" in nature is the doctrine of the "Order of Nine Angles" which is rather jaw dropping in it's advocacy of murder and the suchlike. They were also linked to cases of animal mutilations and were extremely Neo Nazi in their beliefs which also goes hand in hand with modern "Satanic" belief.
Most modern Occultism avoids sch mythical figures as "the Devil" anyway, and is more a form of Psychology with dramatic leanings. Maybe in the US where Christianity is still a huge thing associating oneself with Satan has a certain currency, but in non Religious countries I think it has died out in the same way Christianity has declined and is only used by fans of bad heavy metal to show they are, like, really radical, man.

I rather enjoyed Margaret Murray’s The Witch-Cult in Western Europe and The God of the Witches, which I learned about from John Dickson Carr’s many references to them, even though Murray’s hypothesis (that a powerful pagan cult survived well into the Middle Ages, when the Catholic Church began the witch trials to put down the cult) has been debunked. Strong influence on modern Wicca and on Robert Graves.

Historian Carlo Ginzburg has argued that the benandanti in Italy, who were executed for witchcraft, had their roots in pagan fertility cults, Wikipedia notes. Even if Ginzburg is correct, though, that’s not the same as Murray’s cult that could rival the Church.