Toaru Hikuushi e no Tsuioku

The story revolves around Charles Karino, a Levaamu Empire mercenary aerial pilot who mans the twin seater reconnaissance seaplane Santa Cruz. One day, he receives a preposterous assignment: to fly solo over 12,000 kilometers of enemy waters to protect a girl named Fana del Moral. Fana happens to be the next in line to the empire’s throne and a girl possessing beauty “equal to 5,000 beams of light.”

This is a standalone book, but there are 17 total books in this universe (飛空士).

Adapted to anime as “The Princess and the Pilot”