This was one of my father's old science fiction novels. I originally read it decades ago when (I think) I was in high school, and it made quite an impression on me. I've always meant to re-read it. Finally did.
It held up pretty well. The basic story is that a race of expansionist aliens have their eye on Earth. Their conquest hinges on spies planted throughout society. Humanity's colonies on Mars fall to the invaders in just an hour and thirty-four minutes. And now they're on their way here. The only hope Earth has lies in its leader, Eldin Raigmore. Based on a series of sophisticated tests that gauge people's capacities and slot them into suitable roles in society, Raigmore is one of the youngest of the small elite occupying the highest rank. But unknown to everyone, Raigmore is himself an alien spy.
My younger self was affected by Raigmore's internal conflict: knowing he was instrumental to the defeat of humanity, yet finding himself increasingly human. Today, I don't feel that so strongly, but that may only be because I already knew what was coming. Even so, I enjoyed revisiting the tale. It's not exactly deathless art. The writing is fairly ordinary, and in places its age definitely shows. (Women get judged a lot on their appearance, for one thing.) But the story is still an interesting one, and McIntosh's aliens are very different from us, to the point that their actions in invading seem surprisingly benign (even though we don't want them to win!).
I'm sure this book is long out of print, but if you stumble across a copy, it's worth a read.