I didn't hate it, but this book is very confused about what type of book it wants to be.
Is it a psychological thriller about a physically and emotionally abused woman being gaslit by her abusive husband while she cares for her dying father? Yes, but it resolves so abruptly (after both rape and attempted murder) that it didn't really feel very realistic. (The '80s era German police being very, "whatever, you probably brought this on yourself" did feel pretty realistic though, based on what I know of the German legal system and how it treats rape victims even today.)
Is it a (poorly executed) historical fiction "romance" between a (really horrible) man and his mistress when they get separated by the holocaust? I guess so, but I didn't really have the sympathy for Henryk that I think was the author's intent (seriously, the way he treated his wife Emilie was awful, and I had a lot of sympathy for her, but none for him). Also, when your wife and your mistress are both telling you to GTFO of Nazi Germany before it's too late and you get arrested and set to a death camp, stop being such a whiny, naïve asshat and listen.to.them. (This was several years into WWII, so they all knew about the camps, it wasn't brand new information.)
Is it about an East German translator as she finds her way in West Germany after the Berlin Wall comes down? No, but Eva's story would have been a lot more interesting! The backdrop of Berlin after the wall came down is a great setting that was really squandered.
Is it about the titular Rabbit Girls of Ravensbrück and the evil medical experiments they endured? Also no. Freida interacts with them while she's in the concentration camp, and their story is sad, but the overall plot doesn't really justify naming the book after them (or even including them in the blurb).
Is it about how the Nazis sent political dissidents and academics to concentration camps? Nope. Another missed opportunity. I think everyone knows how the Jewish population was targeted, but the other groups that were killed sometimes get glossed over. I would have rather seen more about the activities sending Henryk and Frieda to the camps over the abuse endured by Miriam.
Throw in some really odd dialog*, an unnecessary and confusing prologue, an unnecessary and confusing epilogue, a "twist" that could be seen coming a mile away, and phrasing that made me regularly think Miriam had reached the last letter when there were actually several more to go, and it was just a mediocre reading experience.
*There is no translator listed, and as far as I can tell the author is most likely British, so... yeah... not sure what's going on there. But some of the conversations just didn't sound like how real people talk to one another.