The Forever Man by Gordon R. Dickson 1988

An ancient starship is found adrift in space, damaged by alien Laagi warships. The voice of its pilot still survives - a mind merged with the ship itself.

Now, Earth's scientists attempt to duplicate the feat. Pilot Jim Wander has been chosen for the dangerous mission. His spirit transferred from his body to his own ship, Wander must make peace with the Laagi...Or lose his ship and his mind.