This anthology (coming ever other book in the series it seems) focuses on Malcador the Sigilite and his plans to influence the Imperiums role in defeating Horus and the insurgency. Some of the short stories are VERY good like Ghost Speak Not and The Sigilitie, some are meh like The Watcher. I don't think it's a must read in the HEresy series to keep up with events, but it is a good insight into the mysterious inner working of Malcador's influence on the Imperium.