JAZZ SINGER Shirley Horn relaxes on a sofa in her Northeast home. A grand piano sits unattended behind her; a turned-down radio gurgles out vaguely familiar melodies. Horn speaks so softly at times that words seem to drip off her lips and fall helplessly into the carpet below. When the spirit pokes her, though, she also can let loose with a laugh that ricochets off the walls.

Dualities exist in Horn's performances as well. There is, first, the empathetic bond between her cool,husky voice and the crisp, clean chords and bright lines she embroiders on the piano. With uncanny ease, Horn chisels languid orchestral lines around her songs. She can purr a ballad, or confess it, or discard it like a second thought.

In the space of 32 bars, Horn can transform herself from a heartbroken innocent to a sultry viper to a worn-out reveler. Her phrasing is precise yet spacious, an inversion of her piano playing, which is relaxed yet strong. Sometimes, the dynamics heat up emotionally, internally; sometimes, they fade away slowly, like campfire embers.

A superb jazz vocalist and pianist who first edged into the spotlight more than 20 years ago, Shirley Horn at 47 is finally justifying the faith of her coterie of fans (including columnist Rex Reed, who calls himself "Shirley Horn's most devoted slave"). This afternoon, she'll perform a celebratory concert at the National Press Club in connection with the release of her second Steeplechase album, "All Night Long." In May Horn begins a three-week residency at Manhattan's prestigious Michael's Pub, followed by a special program devoted to her at New York's Women's Jazz Festival in June. She also has summer dates in London, Nice and the Hague, home of the annual North Sea Jazz Festival.

The North Sea Festival is an important marker in Horn's return to the limelight. Her new album was recorded in concert there in 1981, starting what appears to be a major break after decades of on-again, off-again success. During that time, Horn has benefited from a convergence of kindnesses, concerns and lucky placements; despite a low profile, she has been sought out by folks as diverse as Miles Davis and Quincy Jones, all of them convinced that her talents deserved wider recognition. She is quick to point out that she's never really stopped working--"I haven't been asleep all through these years"-but confirms that the overseas trip last summer was a type of reawakening.

"Even though I had faithful friends here, I had to go to Europe to see how great it is to be loved and admired, appreciated and respected . . . and that's what happened to me. I think it comes at a good time in my life. I realize now that music is a hard business. I'm not full of wide-eyed enthusiasm like I was a few years back. Everybody asks, 'Are you ready for it?' No, I'm not. But I'm grateful. I'm happy that the time is right for me."

"She's beautiful," says Quincy Jones, who produced two Horn albums in the early '60s. Jones, speaking from his California studio between joint sessions with Michael Jackson and Paul McCartney, said: "To me she was the prototype for what Roberta Flack ended up doing. Sometimes you can end up being a little ahead of your time."

At 13, Horn was working the intermission shows at Olivia's Patio Lounge. At 26, she cut her first album, "Embers and Ashes."

She had started piano lessons at 4, encouraged by a mother who foresaw a career on the classical concert stage. She won a citywide talent contest that earned her 13 weeks on a local radio show; she won a scholarship to Juilliard, but couldn't afford to go; instead she attended Howard University's School of Music. "But even before that," she says, "I kind of knew I was going to go into jazz."

"Embers and Ashes" got a great deal of attention, particularly from one jazz giant. "I was at my mother-in-law's house in the country having breakfast and the phone rang. Somebody said he was Miles Davis and could I come to New York?" Unconvinced, Horn hung up, but the trumpeter was persistant, sending her a ticket to New York. "When I got there, to let me know he was serious and knew about my music, he had all his kids singing songs from "Embers and Ashes."

Davis forced Horn's New York debut by refusing to play at his normal club, the Village Vanguard, unless she was hired as well. She remembers opening night as if it were two weeks, not two decades, ago. "I was so excited and nervous. Not only am I playing on the same stage opposite Miles Davis, but in walk Lena Horne, Claudia McNeill, all these great musicians. I'm just looking at the faces. I was passing the bar and Sidney Poitier stopped me and asked, 'Miss Horn, would you like to have a drink?' I was so thrilled, I almost fainted."

Soon after, Horn signed with Mercury Records, and the roller coaster took a little bit of a dive. The label seemed uncomfortable with the image of Horn sitting behind the keyboard; "they envisioned me as a stand-up singing artist." But she always had accompanied herself, and it had become as much a part of her as her clothing; "Basic, that's what I'm all about."

When Horn finally went into the studio, not only was there a new piano player, but also a 44-piece orchestra. So much for her intimate style. "I wasn't comfortable. I was back in the booth and I kept hearing this piano player and I wanted him to play something else."

At Mercury, Horn cut two records and then sat out a five-year contract, by which time the music scene had abandoned her style of singing in favor of rock 'n' roll. "If you listen to those albums,"--"Shirley Horn with Horns" and "Loads of Lovin' "--she says, "you can hear me developing as a human being."

Quincy Jones and Sidney Poitier reappeared in 1968, when Horn was asked to do the theme songs for Poitier's "For the Love of Ivy" and Laurence Harvey's "Dandy in Aspic." Following years of clubwork, she jumped at the chance to work in Hollywood. As it happened, "Aspic" was Mia Farrow's big-screen debut and she had just broken up with Frank Sinatra. So who should be in the recording booth watching Horn? Frank Sinatra.

"That didn't help," Horn laughs heartily. "I'm saying 'Go, go.' When Sinatra left, he waved. Next morning he was in the booth again and when he left he said 'Very nice, very nice, Miss Horn, thank you.' I almost fainted."

Horn spent most of the '60s and '70s in the particular conflict that confronts many women musicians. "I got married, had a baby. I tried to make the scene, but I was torn between my love for my child and, not what you call duty . . . but I came from a very old-fashioned family where a woman's place was in the home."

This was the "beginning of bad times," Horn says now. "I never got out of music, but I got more into working around the house, and the time wasn't right for my kind of music. There were different tugs, directions, depression. The music wasn't always right when I did play." Around town, where she performed at a Southwest lounge called The Place Where Louis Dwells, she also had to battle her reputation as a temperamental and uncompromising perfectionist who could be very tough on sidemen. As her albums went out of print--the last had come in 1971--Shirley Horn was the diamond sliding quietly back into the dark mine. People knew her locally, but her name had pretty much been forgotten elsewhere. Except by drummer Billy Hart, who had apprenticed in a Horn trio as a 17-year-old before going on to international fame.

"In 1980, I got another phone call in a heavily accented voice asking if I'd like to record," Horn remembers. "I thought, 'Oh, another of my friends having fun!' " But Hart called back, offering a contract for the owner of the Danish Steeplechase label. The first call was on Friday; on Sunday, Horn headed to New York, asking her husband to drive slowly so she could figure out what to record. The album, with Hart and bassist Buster Williams, got five stars in Downbeat magazine.

Then the breakthrough trip to the Hague, another serendipitous development. Horn dropped in unannounced at the 1980 Jazz Times convention at the Shoreham. When her bass player and drummer were spotted nearby, she was invited to do a quick guest set, after which the promoter of the North Sea Festival invited her over on the spot. "That was a happy night for me," Horn confesses, "because I got a standing ovation from all the musicians I love."

The ovations followed her to the Hague and onward.So Shirley Horn is back, not with a vengance, but with a philosophical shrug and an optimism the past 20 years have not always justified. "I haven't had any disappointments in music," she insists. "I can sit back and wait. I feel that what's for me, I'll get."