This is the cover of Sylvia Plath’s collection of poetry Lyonnesse. It was posthumously published by the Rainbow Press, a private press founded by Ted and Olwyn Hughes, in a limited edition of 400 copies. Its endpapers contained facsimile reproductions of Plath’s manuscripts.
While her poems had appeared in many magazines and literary journals, Plath only published one collection of poetry in her lifetime, The Colossus (1961). After her death, her estate was controlled by her husband Ted Hughes, who decided to publish her uncollected and unpublished poems in several volumes. In 1971, the same year in which Lyonnesse was published, Hughes also edited collections of poetry by Plath titled Winter Tress and Crossing the Water, both published by Faber.