Master of The First is a short story, available both in book and audiobook formats. After The Lion has gone with most of the Dark Angels forces, the ones that stay at Caliban with Luther are dividing between those that support the decision, and those who begin to think they've been left away and things should't be that way.
Melian, a space marines captain, goes to speak with former chapter master Astelan about his reserves on how some fellow about how the Calibanite marines speak of those coming from Terra, to warn him and try to find some help regarding this seemingly rebels. Will he find some help?
Short, but as most Dark Angels stories interesting due its plot, so that should make you decide if worth it or not.