This book by Chris Roberson tells the tale of some Imperial Fists as they go from being "regular" humans through their induction into the Fists, their time as scouts and their eventual elevation to full battle brothers.

This book is interesting because in the book Heroes of the Space Marines which was published a year or so ago, we see the main characters in the short story Gauntlet Run.  So this first story falls somewhere in the timeline of the novel, but isn't really discussed.