The Forgotten Soldier by Guy Sajer - I finished this book recently and it was great perspective on a front we were not involved in.
Lions of Kandahar - one of the first military books I read, and to this day I am amazed by what one ODA team can do.
Lastly, I recommend you read The Strenuous Life. It is not a book, but a speech given by Theodore Roosevelt, but it motivates the hell out of me.
“I wish to preach, not the doctrine of ignoble ease, but the doctrine of the strenuous life, the life of toil and effort, of labor and strife; to preach that highest form of success which comes, not to the man who desires mere easy peace, but to the man who does not shrink from danger, from hardship, or from bitter toil, and who out of these wins the splendid ultimate triumph.” Theodore Roosevelt