Spirtual and religious beliefs are also an underlying theme in many mystery novels. Dan Brown's novels are one clear example of this kind of pattern. In The Da Vinci Code and Angels and Demons, Brown explores different sets of beliefs within the Catholic Church. He also does an expert job of showing the reader how those beliefs influence the characters. That kind of effect is also brilliantly explored in Tony Hillerman's Joe Leaphorn/Jim Chee series. In Skinwalkers, for instance (the first novel in which Chee and Leaphorn work together), a series of murders is believed to be the work of Navajo skinwalkers, who according to tradition, are supernatural beings (similar to the western concept of witches) who can assume different animal forms, and who prey on humans. Several of the characters in the novel choose not to cooperate with Leaphorn and Chee because of their fear of skinwalkers. Chee himself is put in real danger towards the end of the novel when he is asked to perform a Blessing way (a Navajo cleansing/healing ceremony) - a request that turns out to be a trap for him.
Some authors explore the theme of how a traumatic past affects a character. That's evident, for instance, in Carol O'Connor's Shell Game. I don't want to spoil the novel for anyone, so suffice it to say that in that novel, there's a very clear connection between the murderer's past and the murderer's actions. Thomas Harris' Hannibal Lecter novels are an even clearer example of the underlying theme of characters being affected - sometimes twisted - by what's happened to them.
One theme that I see in my own Joel Williams novels is ambition and its effect on people. Several of the characters in Publish or Perish are willing to do almost anything to get what they want. For one, it's a relationship with the victim. For another, it's a coveted academic fellowship. For another, it's tenure. They sacrifice relationships and ethics and one of them is willing to sacrifice the victim, Nick Merrill. The same theme appears in B-Very Flat. In that novel, Serena Brinkman, a young violin virtuosa, stands between several of the characters and the achievement of their goals, which include a rare Amati violin, financial security (and family money) and a military career.
What themes run through your favorite series? How do those themes add to the novels for you?