Hand of Zei — L. Sprague de Camp

Dirk Barnevelt knew he wasn't a hero, but somebody had to find the explorer who'd vanished on the low-tech planet, Krisha and to do that, somebody had to single-handedly rescue a beautiful princess from bloodthirsty pirates and to do that, somebody had to lead the fleet against an impregnable fortress, while fending off a superior navy. And whoever managed all that was going to be a hero -- whether he knew it or not!