If you have any interest in the Clans, this is the book for you. It contains a detailed history section which spans from the early days of the Amaris Coup, through Aleksandr Kerenksy's Exodus, the Exodus fleet's arrival at the Pentagon worlds, the Pentagon civil war, the Second Exodus, the formation of the Clans, the planning and execution of Operation KLONDIKE, and the aftermath of the conflict. Then, each of the five planets' military campaigns are described in detail (although many details are left ambiguous for the purpose of scenario creation). Additionally, there are sections depicting the important personalities of the period, an overview (and planetary maps) for each of the Pentagon worlds, a summary of each of the twenty Clans, and a section of rules for scenario making, special abilities, and new 'Mechs (and their record sheets, which are available in printable PDF form directly from Catalyst Game Labs).
My favorite part of this book is seeing the genesis of each Clan's "present day" quirks and personalities, such as learning why the Fire Mandrills are so divided or why the Smoke Jaguars were so brutal.
Highly recommended for all die-hard Clan fans or for anyone who really wants to get in the head of their enemy and learn what made them what they are.