The book of Mormon, distinct from the volume as a whole that takes its name, is the point in the record where Mormon stops abridging earlier writers at a distance and narrates events he witnessed and, in large part, participated in directly. Mormon identifies himself as a Nephite general placed in command of his people's armies from a young age, and the book describes the escalating wars between Nephites and Lamanites that consume the final generations of Nephite civilization, wars Mormon leads and, in his own account, eventually refuses to lead further once he judges his own people beyond correction.
This shift from abridger to eyewitness participant changes the character of the text noticeably: earlier books in the Book of Mormon read as compiled history spanning centuries, while Mormon reads as a personal, often anguished account of a specific and final catastrophe, written by someone who expects not to survive it and addresses his account directly to future readers rather than only to his own contemporaries.
Mormon also describes his own reluctant return to command after initially refusing to lead an army he judged too wicked to fight for, an unusual moment of an ancient narrator explaining his own change of mind rather than simply reporting events, which gives the book a self-reflective quality largely absent from the earlier, more compressed abridged material.