To know the history of the Sikh religion, one must first become familiar with Gurbani in the Scripture, Guru Granth, for, here is where one would get the feel for the Philosophy of the Sikh religion as taught, preached and lived by Guru Nanak Dev and also by the remaining 9 Gurus. The great Bhagats, the Bhatts and a few Sikhs had the similar approach and Philosophy and that is why Guru Nanak Dev Ji collected their hymns from different places while on his tours of India.
The Bhatts became devout followers of the Gurus after they observed the lives of the Gurus and their practices in life. Writings of the Bhattas also found place in the Scripture when Guru Arjan Dev Ji prepared the first copy of the Granth with the help of the scribe Bhai Gurdas Ji, in 1604. This Granth was called " Pothi Sahib " and later on this changed to be called the Aad Granth.
History of the Sikh religion was neither written by the Gurus nor by their followers. It was infact written by the adversaries of the Sikh religion. This is why reading the Scripture before reading any of the history books on the Sikh religion is essential.
After one has a good understanding of Gurbani and the doctrines of the Sikh religion, one should read the life histories of the Gurus, Bhagats and Bhatts while sorting out and separating the truth based on their understanding of the Scripture.
This is how one would ultimately become familiar with the real history of the Sikh religion.