BahÃ ' Ã s also believe that the Bible , the Qur ` an , and other books from other religions are special , but do not read them as much as they read the writings of BahÃ ` u 'll Ã h and ` Abdu ` l-Bah Ã . ` AbbÃ s Effendi was BahÃ ` u 'll Ã h 's eldest son , known by the title of ` Abdu ` l-Bah Ã ( Servant of BahÃ ) . His father left a Will that appointed ` Abdu ` l-Bah Ã as the leader of the BahÃ ' Ã community , and designated him as the `` Centre of the Covenant '' , `` Head of the Faith '' , and the sole authoritative interpreter of BahÃ ` u 'll Ã h 's writings .