I don't know what to say except it was a beautifully written novel given the subject matter. It's never easy to write about this particular period in history and never will be. However at least what an author can do is make it readable and make it a good story worthwhile to read.
You really do feel for Stasha and Pearl once they're herded into the camp and are used as experimental fodder to play with. You see both of them mature rapidly and have their childhoods robbed from them near the start of the novel. They were already close to begin with yet because of the circumstances they're closer with them trying to hold and support each other. It's almost heartbreaking to read because without one, the other just simply feels they don't exist.
As to when Pearl disappears, you feel the separation anxiety as you progress through the novel. You feel Stasha's pain and emptiness. Her other half is gone and she has no idea if she's alive or not. You can feel the void within Stasha and as you continue reading, you're still feeling the pain and you're wondering throughout the novel if she will ever see Pearl again. This is great writing on the authors part as you can distinctly feel what the characters are feeling throughout the novel.
There's a small cast of characters in this book. Some stand out more than others. Bruna stood out for me a lot. I loved every aspect of her and her strength. Then you have Peter, Feliks and the nursing staff at the camp. You don't get attached to them as much as Pearl and Stasha are the main ones to be focused on. However, for me, I really loved Bruna.
The only criticism I would have for this book is I found it sometimes a little too wordy and poetic at times. It made it for some areas of the book hard to follow - it would be best to avoid this type of writing. Yes it sets the mood and makes it melancholy but the subject matter itself is already sad and tragic to begin with. I believe that's enough as it is.
Definitely recommended for those that are interested in this particular historical period.