Eternal life
The couple that Holly determines has been kidnapping and killing the women across the small town do so in a quest for eternal life and eternal youth. The only way they are able to achieve this is through kidnapping and killing the aforementioned women.
It is a natural human feeling to want to live forever; many people spend their fortunes searching for a fountain of youth or a way to live forever. Invariably, those people are evil or have bad intentions. The couple in Holly is no different. Their quest for eternal life and youth imbues them with rich characterization and ultimately foreshadows the couple's culpability in the disappearance of the girls that Holly is investigating. In that sense, King's novel is also a commentary on the foolishness of seeking eternal life. After all, there is always a price to be paid for it—often in blood.
The search for the truth no matter the cost
Naturally, one of the most significant themes in a mystery novel is the search for the truth, no matter the cost. In Holly, Holly is tasked by the residents of a grieving town (especially the family of Penny Dahl, one of the girls who has disappeared) to discover who is kidnapping—and potentially killing—young women. One of Holly's strongest beliefs is that people shouldn't rest until they find the truth. The truth needs to be found no matter the cost. If the truth isn't uncovered, then there could be terrible consequences, showing the importance of finding the truth.
Wealth
Professors Rodney and Emily Harris are well-off because of their jobs as tenured but semi-retired professors. Even though they are devoted to each other and seem to be respectable people, they use their wealth for a bad purpose: to search for eternal life. Wealth, when properly used, can be a powerful tool for either good or evil. The Harris' use it for evil. However, other people—like Penny Dahl's family—use their money for good. In the Dahl family's case, they use it to stop the Harris' from committing heinous violent acts against women; they also use it to hire Holly, who helps effectuate the change in the community.
