a brief sketch of the Sikhs by Persian writer Sujan Rai Bhandari, circa 1696, translated by Iqbal Hussain:
To be brief, among the followers of Baba Nanak there are many mystics, discoursers, ascetics, and prayerful men. The essence of the worship of this sect is the reading of the verses of their Guru, which they sing with instrumental and vocal music, and with a heart-ravishing chant. They remove from one's mind the anguish caused by worldly attachments and lift the obscuring veil of impediments from the heart. In their eyes, kinsmen and strangers are alike, friends and enemies are the same. They are wholly one with their friends and do not quarrel with their enemies. The faith which this sect has of it's leader is seldom seen in other sects. In the name of their Guru, which they continuously have on their tongue, they consider it a great act of worship to attend on travelers. If a person arrives at midnight, and takes the name of Baba Nanak, though he be a stranger and an unknown person, even a thief, robber, or person of evil conduct, they treat him as brother and friend and serve him in a manner proper to his needs.from the book "Sikh History from Persian Sources", J.S. Grewal & Irfan Habib
the book has some amazing, eye-witness accounts of Sikh history, something that's sorely missing from Sikh sources. Their views of Guru Nanak Dev ji are unanimously positive and admiring. In fact, their writing on all of the Sikh Gurus are surprisingly positive.
i'll post more as i get time.