Central to Lyndall Gordon’s biography of Henry James is an extraordinary scene. As daylight fades over Venice, James drops a dead woman’s dresses into the lagoon. But the garments fill with air and won’t drown, pressing up against the novelist’s gondola. The scene is riveting for what it tells us about James, but also, more subtly, because it serves as a metaphor for secrecy and revelation – and thus for biography, which feeds on the disclosure of what has been thrust from sight.

In Gordon’s life of Emily Dickinson, secrets stand like loaded guns. The biographer’s first concern is to blast open the sentimentalised depiction of Dickinson as a timid spinster, disappointed in love and endearingly daffy. As Gordon shows, this image is at odds with the witty, intelligent voice of her poems and letters. The Dickinson who emerges here is human, passionate and far from sweet. In middle age she startles relatives by conducting an ardent, if unconsummated, affair. Her imagination is energised by death and suffering. She approves of whipping and punishment, and drowns “superfluous” kittens.

Few of Dickinson’s poems were published in her lifetime, boosting her posthumous reputation as modestly ladylike. Gordon suggests it was not Dickinson’s ambition that was deficient but the understanding of the editors to whom she sent her work. The poet herself, who rose before dawn to write, and who rejected inept editorialising, was fixed on Immortality. In one of Gordon’s acts of inspired reading, the bridegroom Dickinson imagined mounting her stair at midnight is fused with her poetic future: “All her life she heard it coming.”

Attentive reading also triggers Gordon’s answer to the riddle of Dickinson’s withdrawal from society. Shyness allied to a thwarted romance is the conventional explanation. Gordon argues that the poet was epileptic. In an era when epilepsy was stigmatised and feared, a sequestered life was a practical choice. Gordon points to poems that pun on “Fitting”, to images of seizure and volcanic bursts, to the dashes that suggest “something nameless breaking through”, to lines such as “I felt a Cleaving in my Mind – / As if my Brain had split – ”. She reinforces her hypothesis with the revelation that Dickinson’s prescriptions show her to have been taking drugs that were used to treat epilepsy at the time.

The other unspoken drama in the poet’s life was her brother Austin’s affair with the beautiful Mabel Loomis Todd. Its scandalous force split the Dickinson family. In Gordon’s retelling, it’s a narrative worthy of James himself with its repressive New England setting and hidden intrigues. After Dickinson’s death, the battle between the two camps coalesced around control of her poetry. Gordon’s detailing of this prolonged antagonism is reminiscent of Janet Malcolm’s brilliant account, in The Silent Woman, of the struggle to own Sylvia Plath.

Gordon displays an almost uncanny ability to enter the lives of her subjects. At moments Dickinson, her red hair bound in a brown velvet snood, flares from these pages like a shock of light. “Abyss has no Biographer”, she wrote. In Gordon, Dickinson has found a remarkable one.