I hope it's a read that provokes you to explore more. You'll have to in order to get at some truth, because a lot of that book is unfortunately crap. Rhodes was attempting some form of criminal-sociology (the book has been strangely marketed over the decades as lurid history and straight up criminology). However he wasn't a historian and his principal sources, such as Jules Michelet's La Sorcière, are fanciful fictions that have in fact been revealed to be outright historical forgeries in some cases.

The accusations of secret rituals/black mass has its origins (ironically) in Roman attempts to discredit early Christians. The trope was absorbed into Christian writing and turned against its own enemies later on. The trope sees significant elaboration and deepening of detail between 1000 and 1230, by which time it is ready to be deployed against heretics, for example by the papacy in the famously lunatic Vox in Rama of Pope Gregory IX. These were the same basis of accusations against the Templars.

For a thorough look at evidence (and history of fakes) of witchcraft such as the black mass (which later becomes 'satanism'), I would encourage looking at Norman Cohn's very readable Europe's Inner Demons: The Demonization of Christians In Medieval Christendom. Cohn uncovered several important forgeries of medieval witchcraft which reset how we look at the Inquisition and witches.