I have been trying to remember a couple of fix-up novels, where I am pretty certain that I read the whole book, but I can only really remember one of the component stories.
What I remember of this novel is there is a vast interstellar war being fought between humans and some alien empire. As the humans' final weapon, they are building a huge battleship to destroy the enemies. It will take twenty years at least to build, any the crew are being raised from childhood at the same time.
I think this part of the story begins with the protagonist of the other sections of the fix-up in his office, talking to a subordinate. They look out the window, at the battleship under construction, and the subordinate makes a seemingly innocuous comment like, "So, they're really going to attack the enemy home world with that?" There is an awkward moment, and the main character says that the subordinate will have to be checked out, to make sure he's not an alien spy in disguise. The main character says that he might as well go along and get checked himself too.
The viewpoint then changes to the main protagonist's son, who is one of the crew being trained for the ship. At the moment, the son is still a child (maybe about age ten); however, the ship is keyed to him as a future crew member, and an actual alien spy somehow goads the boy into letting the spy into the ship.
That's all I remember. I don't remember how the alien spy is defeated, nor do I remember about what happened before or after this sub-story. Can anybody identify this book? It was probably published more than fifteen years ago, but I don't know the date beyond that. Nor do I have any remember anything about the cover.