Gaunt's Ghosts: The Founding

Gaunt's Ghosts: The Founding - Dan Abnett The first 3 novels of Gaunt's Ghosts series (plus one special short story) in an omnibus edition. Shows how they were founded and has good interaction between characters.
Has plenty of action, and being an Imperial Guard novel, there's potentially a cast of billions. Abnett focuses on a few key characters, and throws in the occasional "red shirt" who gets name-checked just prior to being killed.
The Ghosts are a rough-and-ready bunch, and tend to be looked down upon by more established/higher class Guard units. Partly this is through jealousy as they inevitably manage to get the job done that others couldn't.

The action doesn't let up and is a good read from start to finish.