` Abdu ` l-Bah Ã 's will said that only Shoghi Effendi could interpret ` Abdu ` l-Bah Ã 's or BahÃ ` u 'll Ã h 's books and letters . He also was concerned with matters dealing with BahÃ ' Ã belief and practice â '' as Guardian he was empowered to interpret the writings of BahÃ ` u 'll Ã h and ` Abdu ` l-Bah Ã , and these were authoritative and binding , as specified in ` Abdu ` l-Bah Ã 's will . His leadership style was however , quite different than that of ` Abdu ` l-Bah Ã , in that he signed his letters to the BahÃ ' Ã s as `` your true brother '' , and he did not refer to his own personal role , but instead to the institution of the guardianship .