An Emergency in Slow Motion, By William Todd Schultz

A space known as "Artist's Rooms" at Tate Modern is currently showing Diane Arbus's most recognisable vintage images, no less unsettling for their familiarity: circus freaks, identical twins, tarts and transvestites, they are all there, hovering between the innocent and the obscene, the ordinary and the macabre.

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Yet despite the intrigue that has surrounded these images, and the photographer who repeatedly sought them out over a lifetime, there have been relatively few biographies of Arbus.

Perhaps this is due to her estate's ferocious hold on the material she left behind following her suicide on 26 July 1971. A hagiographic photo-book was released a year after Arbus's death, aged 48, which her daughter, Doon, edited with Arbus's lover, Marvin Israel, who found her body curled up in the bathtub two days after she swallowed barbiturates and slit both wrists. An unauthorised biography by Patricia Bosworth later focused on Arbus's depression and sex-life.

This slim book comes on the back of this limited history. Teasingly subtitled, The Inner Life of Diane Arbus, it attempts to trace the jagged psychological journey of a brilliant but curtailed life. Schultz, a psychology professor in Oregon, begins with an apologia of sorts for those who might be anticipating juicy revelations and definitive conclusions. What he gives us is "psychobiography" (his term), or a psychoanalytic explanation of her life, work and suicide, detailing the way in which the darkness in her life exploded into her art, and dragged her down further.

Much of the raw material appears to be drawn from the 1972 picture-book, Bosworth's biography and also Revelations, the book that the Arbus estate published in 2003 to coincide with an exhibition, containing previously unseen artefacts, including her journal entry noting her "last supper" on the night she killed herself.

The focus is on analysis, and new information that Schultz garnered from Helen Boigon, the therapist that Arbus consulted in the last two years of her life. The revelations here relate to Arbus's sex life - casual sex, sex games with Israel and her insatiable need to connect through sex (she even attempted physical intimacy with Boigon, by stroking her leg with a "slimy expression on her face") - as well as reflections on whether she meant to kill herself or if she sought to "punish" the increasingly distant Israel.

Born as Diane Nemerov into a wealthy family, Arbus grew up with a depressed mother and a distant father. Schultz mentions the "little sexual experiment" between Arbus and her brother more than once, though its details remain a mystery. She married her childhood sweetheart, Allan Arbus, with whom she set up a photo agency, mainly working for the fashion industry. She quickly turned away from this to more gritty subject matter, developed a creative loathing for the "artificial" and the "masked".

This preoccupation has left Arbus open to charges of voyeurism; there are those who accuse her of exploiting her subjects in order to capture their despair. Germaine Greer, photographed by Arbus, said she "had never allowed any of her subjects to look good".

Her fans, however, cite her empathy and her search for the honest image. The truth, suggests Schultz, lies at neither polar opposite. The pictures she took ended up reflecting not her subjects' despair but her own. It is a viewpoint that suggests a pathological self-absorption within her art, which Schultz says, led her pictures to be "Arbused" - her subjects were "made to look like Arbus felt".

The book builds up to its final theory that Arbus's art served as an "accelerant" for her suicide. The dark nature of the work sucked her deeper into her personal abyss, says Schultz, drawing a parallel between Arbus's creative depression and Sylvia Plath's. Yet it might equally be argued that Plath and Arbus would have committed suicide anyway: if photography had not killed Arbus, the boredom of being a wealthy housewife might have been the "accelerant".

A small weakness of the book lies in some of its familiar quotes and anecdotes - from Greer, from former subjects, from Arbus, along with Schultz's habit of repeating certain facts. More significantly, it offers absorbing analysis of her life, though I am not sure how far it succeeds in uncovering Arbus' inner life in the way that her pictures can. If all art is covert autobiography, as Schultz suggests, much of what he says is intuitively conveyed by the pictures hanging in "Artist's Rooms", with their simultaneous display of empathy and the lack of it, their evident thrill at capturing the honest and unvarnished and also their horror at having done so.