The Ghost Brigade and Old Man's War by John Scalzi

Two great military sci-fi books by Scalzi! Really glad I stumbled upon him. If you are a fan of military SF (think Lois McMaster Bujold, David Weber, even some early Heinlein) then you are in for a treat with this series.  Old Man's War is the first in the series, but it's the one I actually read the last -- I don't think you need to read them in chronological order, if you don't want to. Highly enjoyable.