To know the history of Sikhism, one must become familiar first with Gurbani in the Scripture, for, here is where one would get the feel for the Philosophy of Sikhism as taught, preached and lived by Guru Nanak Dev and also by the remaining 9 Gurus. The great Bhagats had almost 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, called the Aad Granth with the help of the scribe Bhai Gurdas Ji, in 1604.
History of Sikhism was neither written by Gurus nor by their followers. It was infact written by the adversaries of Sikhism. This is why reading the Scripture before reading history books on Sikhism is essential.
After one has a good understanding of Gurbani, the doctrines of Sikhism, 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 Sikhism.