The deepest roots of Sri Guru Granth are various, but it was Guru Arjan Dev (1563-1606) who first began to build a Sikh holy book by collecting and writing a coherent collection of hymns to God. This first consolidation of Sikh prayer and teaching would itself be enough to give the fifth Guru an important place in history; the current Sri Guru Granth, which consists of the core collected by Guru Arjan plus other sacred poetry added by Guru Gobind Singh, is a truly remarkable book. But the fifth Guru also became the first person to be martyred as a Sikh, and this also plays an important role in Sikh life. The Sikhs had to face early on the problem of being a strong religious minority under Muslim rule, and under a Guru like Guru Arjan, who was very popular in his region and who drew quite a few Hindus and Muslims to Sikhism, this became quite a problem. Under the reign of the highly tolerant Emperor Akbar, this mostly played out in terms of social tension. However, after the death of Akbar, who was succeeded by his son Jahangir, things changed. Like his father, Jahangir's official policy was toleration. Jahangir, however, almost immediately had to deal with rebellions, and for reasons that are not wholly clear, Guru Arjan was accused of assisting rebels. In particular, the major rebel leader Khusrau was supposed to have been welcomed hospitably and blessed by the Guru. Guru Arjan was arrested, but Jahangir himself seems to have hoped to obtain Guru Arjan's conversion to Islam (it was a common approach among Muslim rulers to treat troublesome religious leaders fairly generously in the hope of trying to get their conversion and thus dealing with the problem at the source). Jahangir turned over Guru Arjan to the governor, Murtaza Khan. It is unclear what Murtaza Khan's instructions were. It's likely that Jahangir authorized the death penalty, but it's not clear that he demanded it and, again, he may have been hoping to get a conversion instead; likewise, it's possible he authorized torture, but there's no actual evidence that he did. There are other possibilities; the exact historical details are somewhat unclear because we have several different accounts from different sources, any one of which could be true. Whatever the case, Murtaza Khan tortured Guru Arjan by hotseat: the Guru was made to sit on a metal seat underneath which there was a fire that blazed to make the metal grow hotter and hotter. According to Sikh stories, which, allowing for adornment are fairly probable, the Guru, after several sessions of this blistering torture, was granted a respite, and allowed to bathe in the river; he entered the river and never came out, his body never to be recovered. Guru Hargobind Singh took leadership of the Sikh community. Guru Arjan died in May 30, 1606, but the first memorial of his martyrdom was on June 16, 1606, and so it has been ever since.
The death of Guru Arjan Dev is in many ways a reasonable starting point for the process by which the Sikhs became not just a religious movement but a cohesive ethnic identity. The Sikhs were shocked by Guru Arjan's death, and this seems to have led to an increasing tendency by the Sikhs to protect themselves by the sword, which led to their reputation as a fierce and well-organized soldier-community; Guru Gobind Singh would later essentially take this element of Sikh life and channel it more fully into religious devotion, leading to the Sikhs as they exist today.
The following is from the hymns, or banis, of the Sri Guru Granth; it is the beginning portion of what is usually known as the Sukhmani Sahib, beginning on page 262, a linked collection of 192 hymns divided into 24 sections of 8, and one of the most popular Sikh prayers. The Sukhmani Sahib itself is quite long; the full recitation can take a couple of hours. 'Gauri' is the type of tune to which it is sung or chanted. The Shalok is a sort of introductory part to the section. An Ashtapad is one of the eight hymns in each section. This Ashtapad is usually attributed to Guru Arjan Dev himself.
Gauri Sukhmani, Fifth Mehl.
One Universal Creator God. By The Grace Of The True Guru:
I bow to the Primal Guru. I bow to the Guru of Ages.
I bow to the True Guru. I bow to the Great, Divine Guru. ||1||
Meditate, meditate, meditate in remembrance of Him, and find peace.
Worry and anguish shall be dispelled from your body.
Remember in praise the One who pervades the whole Universe.
His Name is chanted by countless people, in so many ways.
The Vedas, the Puraanas and the Simritees, the purest of utterances,
were created from the One Word of the Name of the Lord.
That one, in whose soul the One Lord dwells -
the praises of his glory cannot be recounted.
Those who yearn only for the blessing of Your Darshan -
Nanak: save me along with them! ||1||
You can hear this sung -- at this point it is a bit more chant-like than the full song of some parts -- here.