Lots of good suggestions - here are some of my favorites (beyond some of those the have been already mentioned):
The Defense of Duffer’s Drift by E. D. Swinton (a must have, but out of print again

)
The Defense of Hill 781 by James R. McDonough (Swinton with a modern twist)
Steel Wind - Colonel Georg Bruchmuller and the Birth of Modern Artillery by David T. Zabecki
Panzer Battles by F. W. von Mellenthin
Panzer Leader by Heinz Guderian
Lost Victories by Erich von Manstein
Fiction:
Any of the Hammer’s Slammers stories by David Drake
Any of the Horatio Hornblower stories by C. S. Forester (Forester did it first and best :!: )
The Captain from Connecticut by C. S. Forester
The General by C. S. Forester