"The Lost Fleet" by Jack Campbell is interesting in its the space battles and the three dimensional way in which the author describes them that causes you to eagerly turn the pages.
"Omega Force" by Joshua Dalzelle is basically the A-Team in outer space. Similar to Guardians of the Galaxy maybe, but the six main characters are what drives the story. I can't think of any other series of books where I enjoyed the characters as much as these.
If you want a shorter series check out "Old Man's War" by John Scalzi
