Hassall was one of Brooke's Literary Trustees, and contracted in 1959 to write a `Life and Letters' of Brooke. He was able to use the collection of letters which had been compiled by Sir Geoffrey Keynes but not published due to opposition from the Trustees. The biography was finished at the time of Hassall's death, and was seen through the press by Keynes. It appeared in 1964 as Rupert Brooke; a Biography. Keynes' edition of the letters followed in 1968