Easy Does It : WHITE BUTTERFLY <i> By Walter Mosley</i> , <i> (W.W. Norton: $19.95; 272 pp.)</i>

Watts, 1956. Young women of easy virtue are being murdered and mutilated in especially repellent fashion. The police and the press pay little attention, as long as the victims are “Negroes,� but when a young white woman is similarly killed, the powers that be demand action.

Problem is, the powers that be have little entree to the neighborhood.

Sounds like a case for Easy Rawlins--the unlicensed, unofficial and very off-the-books black detective who makes his third appearance in Walter Mosley’s “White Butterfly.�

Mosley (born in Los Angeles but living now in New York City) introduced Ezekiel (Easy) Rawlins two years ago in his first novel, “Devil in a Blue Dress.� That original and highly accomplished work, set in 1948 L.A., became one of the most heralded recent debuts in the mystery-fiction field. A second entry in the series, last year’s “The Red Death,� picked up Rawlins’ progress in 1953 and maintained the high standard set by its predecessor.

Rawlins, a Texas native transplanted to Watts, is in “the business of favors,� doing a lot of things that private eyes often do: finding missing relatives, tracking stolen money, settling disputes. He has a reputation for fairness within his community, which makes him valuable to the police and other authorities. Such types usually lace their requests with threats when they come to ask Rawlins’ help. Generally, it’s the threats that persuade him to assist.

The Easy Rawlins we meet in “White Butterfly� is a bit older, a bit sadder but not much wiser. Married now to a woman named Regina and acting as father to two young children, Easy--the nickname carries a full load of irony--finds it hard to reveal his private thoughts and deeds to those who share his life. He has cultivated concealment as a method of survival in a hostile environment, a world where white men bend the rules to their own advantage and where even the closest relationships can terminate or turn violent without much warning.

For Easy, secrecy is a way of life. He hides his covert ownership of several buildings by posing as an employee of the man who fronts for him--Mofass, the cigar-chomping, chronic-coughing “partner� who has betrayed him in the past but whom Easy has no choice but to stay with. He has only one friend he feels he can count on: Raymond (Mouse) Alexander, a psychopath with a tripwire temper who could as quickly shoot Easy as anyone else.

It’s almost a cliche for African-American males to appear in crime novels as powerful but benevolent sidekicks to tough-but-not- that -tough Caucasian heroes. Hawk, companion to Robert B. Parker’s private eye Spenser, is the best-known example. Easy Rawlins, too, has a hard-nosed black pal he takes with him down the meaner streets. But instead of being a sort of hit-man-with-a-heart-of-gold, like Hawk, Easy’s buddy is a dangerous killer, just this side (if that) of insane. Can it be mere coincidence that this role-reversing predator wears the meek nickname of Mouse ?

In the sunlit danger zone that nurtures such creatures, Rawlins keeps his own counsel, getting solace and dangerous energy from liquor, rarely able to anticipate the next curve. His own wife’s feelings are as great a mystery to him as any whodunit. He is puzzled and frustrated when Regina tells him she wishes he would need her emotional support: “I knew that a lot of tough-talking men would go home to their wives at night and cry about how hard their lives were. I never understood why a woman would stick it out with a man like that.�

Much of the appeal of this series of books lies in their vivid descriptions of the places and people Rawlins encounters daily; this is a part of Los Angeles that Raymond Chandler’s Philip Marlowe knew about but rarely visited. Mosley turns the hard-boiled L.A. novel upside down and inside out, and he does it with prose that seems both spare and exotic--like a palm tree against a flat horizon. His sentences pull the reader along from one bizarre scene to the next, like a spectator in one of B. Traven’s paranoid adventure tales.

Easy soon discovers that Robin Garnett, the young woman whose death has alarmed the L.A. Establishment, was more than the sometime-coed her family knew her to be. Daughter of a city prosecutor, Robin led a double life as Cyndi Starr, a striptease artist known as “The White Butterfly.�

Everything indicates that Robin/Cyndi and the other women all were killed by a mentally disturbed bruiser from Northern California; but there are things about the Garnett case that bother Rawlins, especially after he visits her parents’ home. In Rawlins’ world, things often turn out to be even more corrupt than they first appear.

Like the two novels before it, “White Butterfly� is a pleasure to read, full of well-crafted passages and effective set pieces. As in “The Red Death,� the finale is somewhat disappointing, and this weakens the impact of the book as a whole. Lacking much prelude, the shocking surprise at the end of Easy’s journey does not seem quite credible, even in a context of nearly unremitting moral decay.

But it is the narrator’s perceptions and his own travails that linger in the reader’s memory. You finish “White Butterfly� with a real desire to learn what will happen next to Easy Rawlins. That’s a good feeling for a series character to provoke.