The Book of the Iron Fist contains the history of the men and women who became Iron Fist in a certain time of need. The book along with its pages are fashioned from the scales of Shou-Lao the Undying himself, therefore it can only be understood by those who embraced and correctly control his chi.
It only glows when an Iron Fist touches it because they all draw from the same original power source. The Book of the Iron Fist contains all of the kung fu techniques of every Iron Fist who ever lived in addition to their stories, legends, and mysteries. For untold centuries, the Book was held in K'un-Lun and protected by Lei Kung, the Thunderer who held it in trust for each Iron Fist.
When Orson Randall sought to end the cycle of the Iron Fist; he stole the book and went into hiding. However when his fellow Immortal Weapons came looking for him, he killed one of them in a panic he killed one of them and labelled himself a criminal.
Orson Randall later passes the Book of the Iron Fist onto Danny Rand, his successor and unwittingly the target of the vendetta brought upon him by Orson's killing of another Immortal Weapon. Randall fears that it will be necessary for Rand to master the kung fu secrets of the Iron Fist in order to survive the upcoming Tournament of the Heavenly Cities.
Following Orson's death, Danny has come to plumb the secret histories and stories within the Book of the Iron Fist for answers to K'un-Lun, the Tournaments of the Heavenly Cities, his own father Wendell Rand, and for the tragic and mysterious deaths of the previous holders of his namesake of the Iron Fist.