Sara Paretsky draws upon the tradition of other writers of American detective fiction to craft the character V.I. Warshawski. When one thinks of American detective fiction, two names immediately spring to mind: Sam Spade and Philip Marlowe. Whether it is due to the popularity of the novels or the movies in which they appear (both portrayed, not coincidentally, by Humphrey Bogart), these two hard-boiled detectives are far and away the most well-known.
Sam Spade: Sam Spade is the main character of Dashiell Hammett's novel The Maltese Falcon. Described as "satanic-looking"  he is the quintessential American private eye. He goes to great lengths, risking life and limb, to solve the murders of Captain Jacobi, Floyd Thursby and his partner, Miles Archer. His cool, almost cynical demeanor allows him to catch those who try to evade his justice.
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There are several similarities between Hammett's The Maltese Falcon and Paretsky's Tunnel Vision. First, both contain the strong suggestion that "money is the root of all evil."  Just as people are willing to kill to get their hands on the (supposedly) precious Maltese Falcon, the three conspirators in Tunnel Vision are willing to kill Deirdre in order to keep their illegal dealings with Iraq in business. Also, like Spade, Warshawski is willing to give up personal comforts in order to bring bad guys to justice. She suffers blows to the head, burns, and loses her boyfriend Conrad in order to crack the case. Just as Spade resists the Brigid's attempts to seduce him in order to perpetuate justice, Warshawski resists Conrad's pleas for her to stop. Finally, both Hammett and Paretsky employ a similar fast-paced narrative style, with lots of dialogue. By doing so, their protagonists constantly seem to be "on the move"  This style constantly engages the reader, providing an edge-of-seat reading experience.
Philip Marlowe: Philip Marlowe appears in several novels by Raymond Chandler, most notably in The Big Sleep. He is very similar to Sam Spade in that he is cynical, hard-boiled, eschews the advances of women in order to pursue justice. However, he is a heavy drinker and more prone to violence than Spade.  In The Big Sleep, he solves the murders of Owen Taylor, Arthur Geiger, Terence Regan, and Joe Brody through gritty detective work and spot-on intuition.
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Sara Paretsky herself has stated that she does not want V.I. Warshawski to be interpreted as some sort of "Philip Marlowe in drag."  However, one cannot help but notice some similarities between the two characters. For one, they both enjoy to drink, though Warshawski manages to avoid doing so in excess. Like Marlowe, Warshawski offers quick-witted repartee to those around her, and is hardly ever without an incisive, snappy comeback.
For much more information on hard-boiled detective fiction and American detective fiction in general, I recommend you visit Hard-Boiled Detective Fiction, a website created by William Marling of Case Western Reserve University. In it you will find a great deal about not only the Hammett's and Chandler's, but other writers of excellent hard-boiled fiction, such as Mickey Spillane, Elmore Leonard, James M. Cain, and Sara Paretsky herself.