W. B. Yeats: A Life, Volume II: The Arch-Poet 1915-1939

There are several biographies of the great Irish poet to choose from, and the one you'll prefer depends on how much biography you want. Subtitled "The Apprentice Mage, 1865-1914," this is the one for completists (though they'll have to wait for Volume Two to get through Yeats's death in 1939). The author, a noted Irish historian, renders Yeats's life almost day to day, giving a particularly lively sense of the helter-skelter nature of his early years and a nice depiction of his tumultuous engagement with the Abbey Theatre.