Ian Douglas - Marines in space. Three trilogies; Heritage, Legacy, and Inheritance. Pretty good stuff
Jack Campbell - Lost Fleet series. Pretty good too.
Mike Shepherd - Kris Longknife series. Very good, female Marine. Edit; Oops as a former mud pounder you will have to forgive me for mixing up the squids and the jarheads. First story leads off with her leading Marines as a navel ensign.
Rick Shelley - A bit dated but good boots on the ground stuff. His Dirigent Mercenary Corps series is very good and he has done the Second Commonwealth War trilogy and the 13 Spaceborn - Lucky 13th trilogy. He started the Spec. Ops series before his death.