Tilting The Balance Harry Turtledove

In the second installment (of four), Turtledove returns to the same characters, picking up the stories mere days after he left them in the first book (the first two books comprise just over a year's time). Without giving away too much, even after finishing this book it is difficult to say which way the balance is tilting. The Race is beginning to learn to cope with warfare on Earth (Tosev 3), and is waging victorious war against the Germans, Soviets, and Americans. However, the humans are beginning to understand bits of captured alien technology while the Soviets, Germans, Japanese and Americans are all making reasonably steady progress towards their goal of creating an atomic bomb.