The book is "a detailed guide to the major Houses that inhabit the world of Game of Thrones, filled with information on each House's sigil, history, home, family tree, character profiles, weapons, costumes, and more." The book is fully illustrated and features photography from the HBO series, Seasons 1-5.
The book is a guide to the eleven ruling Great Houses of the Seven Kingdoms that appear during the TV series itself. There are nine major regions in the "Seven Kingdoms", because it also includes The Riverlands (a border region) and The Crownlands (the capital region). Each of these regions has its own ruling Great House. During the course of the TV series, House Stark and House Tully are defeated, after which House Bolton and House Frey are elevated to replace them, so they are considered "Great Houses" by this book and included as well. During the war, "House Baratheon" also fractures into different factions - as the book was produced at the end of Season 5, after Renly Baratheon's death in Season 2, it considers these as two different factions which each get their own entry: "Stannis Baratheon's faction" (House Baratheon of Dragonstone) and "Tommen Baratheon's faction" (House Baratheon of King's Landing - really just an extension of House Lannister). As a result the book has a total of twelve sections.
Similar to the online HBO Viewer's Guide for the TV series, it doesn't have individual subsections for the major vassal Houses immediately below each of the Great Houses - such as House Umber or House Karstark. Instead, characters from these vassal Houses do appear in the listing of retainers and allies for each Great House section (i.e. Greatjon Umber is listed as a vassal in the Stark section).
The book does not contain any exclusive artwork or Heraldry not previously released, though it does contain behind the scenes closeup images of major characters' costumes on display.