These two books take place right after the battle of Calth (I forgot the title of the book that details when the Word Bearers attempt to destroy Calth).
They could almost be ready simultaneously, Betrayer is the tale of what happens next with the rest of the Word Bearers and World Eaters, and Mark of Calth is what happens with those 'less faitheful' sons of Lorgar that are left behind on Calth to continue fighting a losing battle.

Will have more later (gotta run to work), but both books have been amazing and quite revealing.