HAVE NO DOUBT as to whether there is anyone worthy to carry on the succession as grande dame of mystery now that only Dame Ngaio (with powers blessedly undiminished at 78) remains from the Golden Generation that include Dame Agatha Christie, Dorothy L. Sayers, Josephine Tey, and Margery Allingham, all born near the turn of the century.

P. D. James clearly has established her legitimate claim to the title on the strength of seven novels. Death of an Expert Witness (Scribners, $8.95) is the latest.

P. D. James is the nom de plume of Phyllis Dorothy White. Her first novel, Cover Her Face , was published in 1962 in Britain when she was 42. She has written her seven novels while working fulltime as a British civil servant, first as a hospital administrator and now in the criminal department of the Home Office. Her experience in the latter post undoubtedly supplied much of the fascinating detail of police lab work in Death of an Expert Witness.

James is quite possibly the best writer in the mystery field today. She is a serious novelist who uses the detective story as a popular framework to make observations on the human condition and human relationships. That is not without precedent - after all, Dickens turned the melodrama of his day into a vehicle to comment on social conditions. And John Le Carre, in Britain, and Charles McCarry, in this country, are writing spy novels to reflect on the dilemmas in today's world.

"I don't see why one can't say something very true about people and relationships and particularly how people behave under the ultimate stress of death while using a form that is obviously popular," James explains about her choice of the mystery genre, often dismissed as an inferior subspecies of literature.

Death of an Expert Witness cannot be easily dimissed. It is a complex study of people pulled apart and together under stress: the murder of an unpopular scientist at a forensic lab in the English fen country near Cambridge. It is a beautifully-crafted novel: substantial, richly textured, with roundness of character, firm plotting, immaculate prose, and psychological insight.

It is a measure of the author's talent that even the minor characters, down to the idiosyncratic locals, take on life. Here, for example, is Mrs. Swaffield, the vicar's wife.

"Mrs. Swaffield perched upright on the edge of the armchair and smiled across at them encouragingly, bringing into the room's cheerlessness a reassuring ambience of homemade jam, well-conducted Sunday schools, and massed women's choirs singing Blake's 'Jerusalem.' . . . It was not, thought Dalgliesh, that she was unaware of the frayed and ragged edges of life. She would merely iron them out with a firm hand and neatly hem them down."

The murder of Dr. Lorrimer, a cold, unfriendly man, permits the author to explore the relationships between her characters. As Adam Dalgliesh, her detective, observes: "Murder was always solved at a cost, sometimes to himself, more often to others . . . It was a crime that contaminated everyone whom it touched, innocent and guilty alike."

Lorrimer's murder touches innocent and guilty alike: the junior scientist bullied into a nearbreakdown; the director, whose nymphomaniac sister had broken off with Lorrimer; the director's secretary, who was Lorrimer's cousin and his heir until he learns of her lesbian relationship; the police doctor, left by a runaway wife to rear a strange daughter and young son; Lorrimer's elderly father, who frets about a missed breakfast and then asks, "Who will look after me now?"; the young clerical aide, the only one who remembers Lorrimer kindly because he encouraged her studies.

And, finally, there is the character of the victim himself: "This was the question that lay at the heart of every murder investigation . . . It was the strangest part of a detective's job, this building up of a relationship with the dead, seen only as a crumpled corpse at the scene of the crime or naked on the mortuary table. The victim was central to the mystery of his death."

James does not forget the element of detection in the fullness of characterization and atmosphere. The plot moves to a satisfying and suspenseful resolution once she has introduced her characters in the slowly-orchestrated first section.

Although Death of an Expert Witness is a solid addition to the canon of James's mysteries, it is not the best introduction to the talents of the author. Far better to discover her in the superb Shroud for a Nightingale or A Mind to Murder. Another prime choice is a delightful An Unsuitable Job for a Woman (in which Dalgliesh, the protagonist of the other six novels, makes only a brief appearance).

In Death of an Expert Witness. Dalgliesh is curiously subdued and contained. He is descended from the blood line of the British gentleman-detective and is both the professional policeman and the published poet. In previous books, James fleshed out Dalgliesh as a complex man - compassionate, introspective, more secure in his work than his personal life and philosophy. He can understand the near-incestuous relationship between brother and half-sister, who share an arrogance of superior intelligence and cultural refinement.

But in the latest book, James seems to assume that the reader already knows Dalgliesh from his previous appearances. So the first-time reader does not see the detective in full portrait as a peer to rival Lord Peter of Roderick Alleyn or Albert Campion.

In carrying on the grand tradition of British women mystery writers, James probes deeper into the complexities of human nature than any of her predecessors. She is a worthy successor to receive the mantle of the grande dame of mystery.