This thesis explores the work of three post-war American poets—Robert Duncan, Elizabeth Bishop and Amy Clampitt—for whom the practice of letter writing was already a disappearing art. In placing these poets and their letters side-by-side, the thesis makes connections between poets who have previously been seen as inhabiting different and largely discrete poetic spheres. The thesis intervenes in the growing field of epistolary scholarship, extending and amending the findings of previous critics who have observed the close relationship between letters and poems. It challenges a recent critical emphasis on letters as sources that should be considered independent from poems, arguing instead that the two art forms are deeply interwoven. Through an examination of particular case studies and detailed close readings of published letter collections and unpublished archival material, the thesis demonstrates how Duncan, Bishop and Clampitt used letters as inspiration and material for their poems. The thesis uncovers a shared lineage with nineteenth-century and earlier letter writing conventions, showing how these poets replicated prior practices including the coterie circulation of poems in letters, an Emersonian concept of friendship, a “baroque prose style” and miniature portrait exchange. For three poets who existed on the margins of various literary movements, as well as often being geographically isolated, letters were a vital source of friendship and companionship. However, in each case, letters were not perfect models of harmonious friendship and community. In fact, the sense of connection created through letters proved to be nearly always, and necessarily, virtual and delicate.