The first half of the book was a history of Salem, Massachusetts, and the surrounding area, in the 17th century. This lays the foundation for what transpired in 1692, the accusation of the practice of witchcraft by 200 individuals, their trials, torture (to induce confessions) and deaths by hanging.* Ostensibly, the motive force behind the trials was that the condemned were doing the work of the Devil, but other motivations - prejudices, ambition, jealousies; petty grievances, long-standing grudges and private feuds - fed into that.
Upham states that the Puritans had inherited two heathen doctrines - the world being ruled by the twin principles or beings of "the perfectly good and the perfectly bad." For Christians, and the Puritans, these became "the Deity and the Devil." Regarding the Devil, Upham writes that "demonology" had a long history in Europe with the Inquisition and that, in the 16th century, immediately prior to the Salem trials, "works of the highest pretension, elaborate, learned voluminous and exhausting, were published by the authority of governments and universities to expand it." Demonology "was regarded as occupying the most eminent department of jurisprudence, as well as of science and theology." Witches were those who had made formal compacts "with Satan to aid in rebellion against God." For the Puritans, Upham writes, the final battleground between God and the Devil was Salem.
The book reveals a lot about the human capacity for evil, but not in the typical way we understand evil. In Salem, at that time, average-enough people were willing to engage in such atrocious practices against their neighbors out of their fear of the Devil and their wanting to do "right" by God, against the background of mass hysteria and conformity ("the credulity and superstitions of mankind"), cheered on by magistrates and preachers and "the love of God."
Literally, the people of Salem lost their minds. Fortunately, in the mid- to late-1690s, the people came to their senses and later - too late - recognized the sins that had been conducted in the name of God.
There's a lot of detail about Salem and the trial making the book somewhat a slog if one were looking just for the overview. For that, the Wikipedia entry worked just fine.
*The AI summary: The Salem witch trials were a series of hearings and prosecutions of people accused of witchcraft in colonial Massachusetts between February 1692 and May 1693. More than 200 people were accused, and 20 were executed, mostly by hanging. The trials were fueled by fear, religious extremism, and social tensions in the Puritan community of Salem.