Written as a history book penned by an unnamed maester of the Citadel, one who clearly serves/served Robert Baratheon and admired him, this book traces the history of Westeros--and Essos, where applicable--from prehistory to the end of Robert's Rebellion. It also includes chapters detailing the cultures of the Seven Kingdoms and rest of the known world, as well as a timeline and several family trees. If you're interested in having the entire history of Westeros laid out for you instead of having to piece it together from A Song of Ice and Fire, this can be extremely helpful. However, like all of Martin's narrators, the maester is not an unbiased source, and thus some of his assertions must be taken with a grain of salt.