She helps Spenser with several of his cases and is his ongoing love interest. Susan is first introduced as a high school guidance counselor over the course of the series, she eventually attends Harvard to obtain her PhD in Psychology. Also central to Spenser's character, and to the plot of A Catskill Eagle, is the detective's relationship with Susan Silverman. Spenser served in the First Infantry Division.

In many ways, Spenser resembles his creator: both, for instance, fought in the Korean War. Although he speaks and postures like the tough guy old school detective that Parker helped elevate to archetype status, Spenser (whose first name is never revealed) also has a cultured, James Bondian side: he is well-read, likes to cook, and is guided by a strong moral compass. He is a former boxer who, famously, once took on heavyweight champion Jersey Joe Walcott. And even if he forever flies within the gorge, that gorge is in the mountains so that even in his lowest swoop the mountain eagle is still higher than the other birds upon the plain, even though they soar.” Detective Spenser's backstory and character emerge in pieces over the course of the many novels Parker wrote about him: he grew up in Laramie, Wyoming, and now works as a private eye in Boston. The book's title comes from the Herman Melville quote, “And there is a Catskill eagle in some souls that can alike dive down into the blackest gorges, and soar out of them again and become invisible in the sunny spaces. Although A Catskill Eagle was never turned into a TV movie, several of Parker's works involving Detective Spenser have been. The story follows Detective Spenser as he tries to save the woman he loves from what seems to be a case of Stockholm syndrome. Parker’s twelfth crime novel featuring Detective Spenser, A Catskill Eagle (1985), is characterized by tight, spare prose and a quick-moving, action-filled plot.

A Catskill Eagle by Robert B. Parker