The private investigator is a 1920s archetype hardy enough to have survived into our new century, a self-reliant hero who adheres to a personal code of honor. Robert B. Parker's Boston private eye, Spenser, is a classic of the type. "Sixkill" (Putnam, 293 pages, $26.95), the 39th Spenser book, was also the last to be written by the prolific Parker, who died last year. (The series will continue with novels by Ace Atkins, according to Parker's estate.)

Ex-cop Spenser, with his office overlooking the Charles River and his psychiatrist-girlfriend Susan, is a fixed point in a changing world: At ease with himself to the point of smugness, he mocks villains with his comic banter and follows duty down whatever path it leads. Why? "Because I can, I guess," he ventures. "Because I'm good at it." To which Susan adds: "And because you want to see things come out right."

The task Spenser has accepted this time is discerning the guilt or innocence of a repellent young movie-and-TV star ("loud, arrogant, stupid, foul-mouthed") in whose hotel room a 20-year-old woman has been found dead. Spenser persists in trying to find out what happened despite the boorish star's intransigence, a threat from the star's agent and the arrival of a sadistic West Coast enforcer bent on writing Spenser's death scene.

'Sixkill' the 39th Spenser book, was also the last to be written by the prolific Parker, who died last year. ?Millennium Images, UK / Contributing Photographer

Aided by the suspect's Cree Indian ex-bodyguard, Zebulon Sixkill (who adopts the P.I. as a mentor and role model), Spenser bulls and breezes along in a predictable manner—but then the predictability is part of the pleasure. And, predictably, the body count is high. "Ain't a lot of happy endings here," Spenser's new sidekick notes, as the duo staves off the black-money interests with a stake in the star's reputation, and does its best to bring about justice for the dead girl. Spenser, though, sees his exertions as a rainstorm washing away the existential grime: "The cleanliness was almost certainly illusory. . . . But life is mostly metaphor, anyway."

Things aren't so simple for Matthew Scudder, the sober, reflective, unlicensed New York investigator who narrates Lawrence Block's "A Drop of the Hard Stuff" (Mulholland, 319 pages, $25.99), the 17th Scudder book. Like Parker, Scudder is an ex-cop; he left the force after accidentally causing the death of a juvenile bystander in a shoot-out. But while Spenser can take alcohol or leave it alone, Scudder dares not have that first drink, lest he tumble into binge-blackout and its dreadful aftermath.

The daily routine of 12-step recovery is at the heart of this moving tale, set in a pre-cellphone era when Rubik's Cube was the gizmo of the hour and AIDS didn't yet have a proper name. When an AA member whom Scudder knew as a youth is murdered, the man's program-sponsor asks Scudder to look at a list of people with whom the victim desired to make amends. Could the killer's name be on it? The job comes just when Scudder is approaching his first-year mark of sobriety. As the P.I's own sponsor says: "Sometimes things get a little crazy right before a person's anniversary." In probing the death of this fellow alcoholic, Scudder puts his sobriety—and his life—in jeopardy.

There are no tough-guy certainties in Scudder's world, no salutary shootouts with villains. Matt is courageous enough, but his brain helps him more than his brawn. The pleasure of this elegiac novel (a book right up there with Mr. Block's best) comes not in its adherence to a formula but in the way it keeps us guessing. Its ending is both unresolved and satisfying—which seems appropriate to its human-size hero, a vulnerable but capable man whose professional and ethical credo is "one day at a time."