Sri Guru Granth Sahib is the Living Guru of Sikhs. Unlike the Most Important Scripture for Other religions, a true Sikh does not view it as a Book. Sri Guru Granth Sahib is regarded the Living Guru. I would quote Baba Nand Singh Ji Maharaj, the highest devotee of Sri Guru Granth Sahib:
"Sri Guru Granth Sahib is the Living, Awake Sri Guru Nanak Sahib who talks"
