Hallowed Ground (Flight & Glory #4) by Rebecca Yarros

There are some debts you can't repay...

Josh Walker is loyal, reckless, and every girl's dream. But he only has eyes for December Howard, the girl he has yearned for since his high school hockey days. Together they have survived grief, the military, distance, and time as they've fought for stolen weekends between his post at Ft. Rucker and her schooling at Vanderbilt. Now that Josh is a medevac pilot and Ember is headed toward graduation, they're moving on—and in—together.

Ember never wanted the Army life, but loving Josh means accepting whatever the army dictates—even when that means saying goodbye as Josh heads to Afghanistan, a country that nearly killed him once before and that took her father. But filling their last days together with love, passion, and plans for their future doesn't temper Ember's fear, and if there's one thing she's learned from her father's death, it's that there are some obstacles even love can't conquer.

Flight school is over.

This is war.

The Flight & Glory series is best enjoyed in order.
Reading Order:
Book #1 Full Measures
Book #2 Eyes Turned Skyward
Book #3 Beyond What is Given
Book #4 Hallowed Ground
Book #5 The Reality of Everything