"Good Sci-Fi Military Book" Topic

I'm currently looking at Legionnaire and Columbus Day. Anyone read either one? Any other book recommendations? Thanks.

Author(s)?

Robert Frezza – A Small Colonial War, Fire in a Far Away Place, and Cain's Land.

What goragrad said. I am presuming the OP has heard of Hammer's Slammers etc.

Goragrad is CORRECT…probably one of the best Mil SciFi set ever. The narrative has it's own unique pacing, but you get used to it quickly.

What sort of flavor are you looking for? Space Navy (or Force), Wet Navy, Ground Pounders? Near future or far future? Hard physics or wild physics?
In the Trenches or Big Strategic Picture? Blood and Mud or Vacc and Atoms?

I've enjoyed:
John Ringo's Hot Gates trilogy. Tongue in cheek, less specifically military, but fun (and essentially he set it in the far distant backhistory for the SF military webcomic Schlock Mercenary).
He also does a bit more "Blood and Mud" with his Posleen novels. (If you like conservative/libertarian politics, you'll love Ringo. If you don't, he still tells a good tale.)

Elizabeth Moon's Vatta War series. Space Navy, loosely following the Hornblower concept. A bit more Vacc and Atoms, and also less "this is life in the ranks," but excellent. Moon is a class above as a writer. Her Serrano series is also good. Slightly less military, but it's still there.

Mike Shepherd's Kris Longknife series. Also Space Navy and Hornblowerish. Lots of space battles, with a few ground scraps. Vacc and Atoms mostly.

John Campbell's the Lost Fleet series. Space Navy, Vacc and Atoms, Big Picture, what's the heroic Admiral gonna do, along with the internal politics of running a fleet trying to fight its way back home through enemy territory. (Shades of Xenophon in space.) Really well-thought, realistic space battles. Far future, but most of the physics is on the hard side.

R.M. Meluch's Tour of the Merrimack series. Over-the-top military Space Opera. Space Navy, but also Marines. Vacc & Atoms with a touch of Mud & Blood. And it's got Space Romans. Seriously.

I confess to being more of a Space Navy, Vacc & Atoms guy in my tastes.

Almost forgot the Destroyermen series by Taylor Anderson. Two WW2 US Navy destroyers enter a strange storm, pursued by a Japanese battleship. When the storm clears, they find themselves in an alternate Earth, populated by intelligent blood-thirsty dinosaurs and equally intelligent sea-going lemurs whom the dinosaurs pursue as prey. The war shifts to this world as the Americans side with the lemurs and the (insane) Japanese admiral sides with the dinos. Battles rage across land, sea and air, with surprises popping up everywhere. None of this is high tech; a big portion of the novels deal with how the two sides adapt or create low-tech weaponry to replace their damaged and aging "modern" tech as the war goes on.
Fun series!

For starships, I would recommend Britt Ringel's "This Corner of the Universe" series.

For groundpounders, try "All You Need is Kill", by Hiroshi Sakurazaka. It formed the basis for the movie "Edge of Tomorrow", and is an excellent work in its own right.

I'm finishing The Forever War. Not as much action as I had hoped for but still found it fascinating. This Corner Of The Universe is next. I'm still considering Lost Fleet, The Ember War and Duel In The Dark. Belated thanks for all the suggestions.

Well, for some dandy space battles I can recommend two of my own books: "Across the Great Rift" and "The Terran Consensus". Available (with my "Great Martian War" series) on Amazon.

Take a look at Glen Cook's Sci-Fi books.

He has written several of them.