The Star Conquerors

I first read this book by Ben Bova as a kid. It's a volume in the wonderful John C. Winston science fiction series for young adults and one of the hardest to find. Bova took the setting in this one and extended it for several more books. Many of those have been reprinted, but for some reason he has refused to allow this volume to come into print again (driving those collector's prices even higher). Set in the future where the small collection of Earth-centered worlds is fighting a losing battle against a mysterious enemy, Bova bases the story on a number of historical events (such as Alexander's conquest of the "known world"). Good space action, some interesting characters and a gripping story (even for a "young adult" book). Come on, Ben, bring it back into print!