The most common assumption about the Church of Satan, then and now, is that its members worship a literal devil. The Satanic Bible exists largely to correct that assumption, and it does so in its own voice rather than through a critic's paraphrase.
LaVey states plainly that Satan is not a being to be prayed to. He is a symbol, standing for the carnal, the earthly and the self, set against what LaVey called the "spiritual pipe dreams" of religions built around denying the body and deferring reward to an afterlife. The book's ethics follow from that starting point: indulgence rather than abstinence, vengeance rather than turning the other cheek, responsibility to the responsible rather than love wasted on the ungrateful. It reads less like a manual for devil worship than like a deliberately inverted sermon, borrowing Christianity's own vocabulary of sin and virtue and running every value the opposite direction.
That inversion is also its clearest organizing idea. LaVey was raised in a culture soaked in Christian moral language and built his religion by keeping the vocabulary and rejecting the content, individualism instead of submission, this world instead of the next, the self instead of a god outside it. Whatever else The Satanic Bible is, in the founder's own stated terms it is not devil worship. It is atheism performed in Christianity's own costume, a distinction LaVey insisted on for the rest of his life.