Hilda Doolittle: 'the purest Imagist of them all'; Herself Defined: The Poet H.D. and Her World, by Barbara Guest. New York: Doubleday. 360 pp. $18.95.

''She is like a person walking a tight-rope. You wonder if she'll get across, '' said D.H. Lawrence of Hilda Doolittle, the poet better known as H.D. She was emotionally wobbly, a woman who weathered the jostling of a world she perceived as inimical only with the assistance of dear friends. She was also bolstered by a sinewy courage gleaned from her work. Like Emily Dickinson and Sylvia Plath, H.D. bolted away from interior demons straight into her craft with such fervor that her verse profited.

Under Ezra Pound's tutelage, H.D. wrote poetry that so beautifully exemplified the tenets of Imagism that Barbara Guest lauds her as ''the purest Imagist of them all.'' Her efforts prodded that nascent movement into the vanguard of modern poetry, though she is now accorded less homage than Eliot, Pound, or Lawrence.

She came from a background conducive to being diligent and cerebral. Born in September 1886 in Bethlehem, Pa., H.D. was nurtured by the teachings of the Moravians, who inculcated their followers with a zeal for hard work. Her father, an instructor of astronomy at the University of Pennsylvania, was detached, remote, completely immersed in scholarly duties. H.D.'s mother was equally inundated by domesticity. Into this dank atmosphere came Pound, then a University of Pennsylvania student who was already a genial maverick. H.D.'s destiny would be yoked to his for 50 years of friendship, through turmoil, war, and separation.

London was the hub of European literary activity during the second decade of the century when H.D. went there. With Pound as her conduit to England's most renowned denizens, she met May Sinclair, Yeats, and others. Yet nocturnal socializing was inconsequential compared with another impending event. Imagism was about to germinate.

Its practitioners scuttled the Victorians' mawkishness and ornamental language in favor of an almost frigidly sparse style. Imagist poetry emphasized primacy of subject matter, conciseness, and adherence to the musical phrase for rhythmic structure. In Pound's opinion, the goal was ''objectivity - no slither.''

''H.D. came to be known . . . as the finest Imagist,'' explains Guest. In her private life, she was far less intrepid than she was as an author. She dodged her problems in order to write, but her equilibrium was always imperiled by the ''inordinate inferiority complex'' that dogged her. To maintain her calm, H.D. snugly braided the lives of many others into her own existence. It was an agreeable symbiotic relationship, because these people reveled in their proximity to H.D. while they nourished her precarious self-esteem.

Barbara Guest has refurbished H.D.'s literary reputation, and it is to Guest's credit that she has nudged the poet toward a more prominent niche in the annals of literature. Regrettably, descriptions of the fertile cultural milieu of the age sometimes detract from H.D., who deserves to be the cynosure. Irritating digressions concern those who strayed into H.D.'s erratic orbit, and in general, ''Herself Defined'' would have been more lucid without this congestion of minor personalities.

H.D. existed within a crucible of torments and did not emerge unsinged, but always, the artist persevered. T.S. Eliot aired his skepticism when he wrote that a poet ''may have wasted his time and messed up his life for nothing.'' As Barbara Guest makes poignantly clear, the disarray in H.D.'s life was not in vain, her faith in her art rarely vacillated, and she got across the tightrope.