The Guru Granth Sahib is organised by raga, musical mode, and this is not an editorial convenience. The scripture's arrangement puts compositions under the mode in which they are to be sung, thirty-one of them, and the tradition holds that the mode carries part of the meaning. Japji Sahib is a notable exception: it stands before the raga sections and is recited rather than sung.
The performance tradition, kirtan, is central to Sikh worship and is carried out by musicians in the gurdwara, and the instruments and the modes are prescribed by tradition. That the scripture arrived with its music attached makes it unusual among the world's scriptures, most of which acquired their musical settings later and from outside.
Two further points about the text as an object. The Guru Granth Sahib contains compositions by figures who were not Sikh Gurus, Kabir, Ravidas, Namdev, Sheikh Farid and others, from both Hindu and Muslim backgrounds, included by editorial decision because they were held to express the same understanding. That inclusion is a substantive theological statement.
And the scripture is not a book in the ordinary sense within the tradition. Authority passed to it from the tenth Guru rather than to a further human successor, it is treated as the living Guru, installed each morning and retired each night, and it is addressed and attended in ways that would be inappropriate to a text. Describing it as the Sikh holy book is accurate and misses what the community means by it.