FOR ROBERT B. PARKER and his Boston-based shamus, Spenser, the last few years have been very good ones. The books have been best sellers; Spenser has his own TV show; and the author has become a celebrity as a gourmet cook, featured on the food pages of newspapers.

There is an irony here: As Parker's popularity has risen, his accomplishment has declined in inverse proportion. The Parker of the recent 1980s often has been a self-indulgent, sloppy writer. And Spenser, who had been fleshing out as a human being in the earlier novels, has been overtough and oversentimental, spending more time at weight lifting and soul searching than the detecting business.

Now, with Taming a Sea-Horse (Delacorte/Lawrence, $15.95), Parker has written his 13th Spenser adventure. It is not an unlucky number. Parker is in better form than he has been in recent books, although he still falls short of earlier Spensers like The Judas Goat and Promised Land, which won an Edgar in 1976. Parker still can tell a story -- even though the plot is thin and covers the old familiar ground of prostitutes, pimps, and the Mob.

Parker's dialogue crackles, and his one-liners are among the best in the business. The narration is bright and breezy. Spenser is a special guy: lover of good food; a wisecracker who can quote, and aptly, from T.S. Eliot about measuring out life in coffee spoons; an ex-boxer with a passion for the Nautilus machine; a private eye with his own moral code who values people over abstractions and the finer points of the law; a macho man with compassion for the victims.

Taming a Sea-Horse picks up the story of April Kyle, the teen-age prostitute and drug addict rescued by Spenser from Boston's notorious Combat Zone in Ceremony (1982). At the end, when April made it clear that she wasn't about to give up whoring as her profession, Spenser took her to a friend who ran a high-class bordello in New York City.

Now April has left this hooker's haven because she believes she has fallen in love with a black musician and wants to help him finish his courses at Juilliard. The madam calls Spenser to New York and warns April is on her way to becoming a street-walker. Prostitutes have only downward mobility.

April is as stubborn as ever when Spenser tries to talk to her. So he tails Rambeaux, the musician, and finds he is a cheap pimp with a string of girls on the street. One of these, Ginger, who already shows signs of wear and tear, has been dumped by Rambeaux as his lover after a replay of April's love-smitten story.

Both Rambeaux and Ginger are murdered, and April vanishes. All Spenser can do is follow Ginger's trail with the hope it will provide a lead to April. The quest takes him to a child-abusing father in Maine, a massage parlor in Portland, a private club in Boston with hostess-playmates for wealthy clients, and a Caribbean resort. With the formidable Hawk at his side, Spenser strikes a deal with the Mob to free April, who has been used to provide amusement for a kinky bank president who launders the Mob's money.

The big problem in Taming a Sea-Horse is April. Spenser may be a champion of victims, but April is such a willing victim it is difficult to care what happens to her. She doesn't seem worth Spenser's risking his hide in a second attempt to save her if she wants to self-destruct.

The Hired Hand

NO Word From Winifred (Dutton, $14.95) is not your conventional mystery. There is a mysterious disappearance, but the central theme is a study of the importance of friendship between women and how this affects their relationship with men. In a recent essay, novelist Margaret Atwood pointed out that this is an area of a woman's life -- she called it Best Girlfriends -- that has been neglected in novels until recently.

Hardly sounds like the stuff of a mystery, you would think. Yet Amanda Cross, whose amateur sleuth is the liberated Professor Kate Fansler, has turned such material into a compelling -- and infuriating -- off-beat mystery novel. Not the least of its attractions are the parallels, tenuous and yet intriguing, to the lives of Mary Renault on the one hand and Dorothy L. Sayers and Muriel St. Clare Byrne on the other.

Amanda Cross (the pseudonym for Carolyn Heilbrun, a Columbia University professor of literature like her heroine) is an acquired taste. Her followers think she is witty and civilized. Her detractors argue that she is an intellectual snob and that one never talks the way her characters do, even in academic and literary circles.

In No Word From Winifred, a reluctant Kate agrees to check into the disappearance of Winifred Ashby, whose "honorary" aunt had been Charlotte Stanton, an Oxford don who wrote popular novels about ancient Greece. Winifred vanishes in London just when she is to help a young woman who is Stanton's official biographer. All she leaves behind is a journal found on a New England farm.

The mystery is two-fold: Where has Winifred been in the years before she reappears in London and why does she vanish again? The journal provides clues in the past, and Kate discovers that Winifred has had a love affair with Martin, another professor, and, at the same time, developed a close and valued friendship with her lover's wife, met by chance at a literary seminar. It is a strange and explosive me'nage a trois.

No Word From Winifred is not for everyone. It starts at a painfully slow pace and often is confusing to follow. Yet the patient reader will be fascinated by Kate's search for the real Winifred through the journal to Oxford to a convention of the Modern Language Association in Houston.

Those who think Cross is too stuffy and highbrow should read her witty observations on academics in convention. It is true that Cross does not have the blithe spirit of Sarah Caudwell, the British writer of wickedly witty, elegant mysteries, but she does have her moments.

In exploring female friendships, Cross, an ardent feminist, has muted the strident polemics that marred her recent books. Instead, she offers a character study of a strong, independent woman who retreats to the life of a hired hand on a New England farm to escape a dangerous triangle when her lover becomes jealous of her friendship with his wife.

Original Sin

IT HAS BEEN three years since that last outing for the Rev. C.P. Randollph, the popular cleric-sleuth introduced by Charles Merrill Smith, a retired Methodist clergyman.

Before his death last year, Smith had nearly completed the manuscript for the Rev. Randollph's sixth confrontation with the sin of murder. It now appears as Reverend Randollph and the Splendid Samaritan (Putnam, $15.95) with finishing touches from Smith's son, Terrence, also a writer.

The posthumous book offers one of the cleric's best cases. The Splendid Samaritan is a cleverly plotted story of long-festering revenge. C.P. (Con) is as engaging as ever as the liberal pastor of a fashionble, skyscraper-housed Chicago church and the husband of Samantha, the beautiful TV talk-show interviewer. (Randollph is another one of those all-accomplished, overachieving sleuths -- he was a star quarterback for the Los Angeles Rams before turning to the ministry.)

Once again Randollph teams with his friend, Police Lieutenant Mike Casey, when a wealthy parishioner is murdered in a brutal, ritualistic fashion. The victim had been dubbed the Splendid Samaritan because of his lavish philanthropy and support of Business Executives for Christ (what fun Smith, the former minister, has with the group's breakfast prayer meeting when Randollph is introduced as "throwing those touchdowns for Christ").

Randollph and Samantha dig into the Samaritan's past and discover he has not been that splendid. The philanthropy covered up the transgressions of a greedy man whose fortune was built on a swindle years earlier. The reader knows the motive for the murders (the Samartian's demise is linked to three other deaths), but Smith comes up with a jolting surprise at the end.

For fans of the series, there is the assurance that Smith's son will continue his father's good works with more Randollph adventures.