@janbartosik Yes, I really enjoyed the book too, and will probably read it again, but there are a couple of important reader caveats:
The book plays fast and loose with the technical details.
There is a risk of cheering for the U-boot crew, despite the fact that their branch was a critical pillar of fascist power.
(Naval officers weren't allowed to be members of the Nazi Party at the time, but the Nazi Party couldn't have held onto power without the support of the U-boots, so ...).