The 'current' timeline drags a lot. Generally predictable plot too. The ww2 part is more engaging but has the good secretly Christian Nazi and the evil godless evolutionist Nazi trope with the brothers. 

The book has a section excusing the Nazi who's secretly Christian. Nazi Germany was an overwhelmingly Christian nation. The book mentions Christianity in general and instead of having the setup of 'the secret Christian Nazi's religion' be because of possible tensions between the majority Protestants among the Nazi's having possible conflict with Catholics and specify he's Catholic or something. The book frames it as him being a secret Christian because Germany was outlawing religion. quote: "I was horrified at what he was doing to the Jews in Germany, and the way he was outlawing religion. 

(continuation of the quote, from the book at around 78%)

Quote: “Even before the war began, I was horrified at what he was doing to the Jews in Germany, and the way he was outlawing religion. I’m a Christian, as you know, a fact I had to keep hidden for my own safety. But by that time, I’d already been chosen to join the intelligence service. I had no choice, Sophia. I would have been shot if I’d refused.” “My Frederik, what you have suffered.” Sophia’s eyes were full of tears.

(the evil godless evolutionist Nazi's evil monologue)

Quote: Falk looked at his brother, then gave a harsh snort of laughter. He laughed so hard his gun wavered and he pulled himself up short. “Ah, Brother, you’re such an idealist! Those poems you used to read as a child—romantic rubbish! Your belief in God, your much-vaunted intellect and skill at philosophy, when you do not see what life is really about. Life is cold and hard and cruel. We do not possess the soul you have always talked about. We’re nothing better than ants who crawl around blindly across the planet. You have never understood reality. It’s dog eat dog in this world, Brother. Everyone for themselves. You think your little life matters—or hers? You really believe that love”—Falk spat the word out—“can conquer all? You’re deluded, Frederik, as you always have been. And now it’s time I taught you what reality is about.”