I just finished Katya and it is great!!! It is fascinating and suspenseful. I started reading it yesterday evening when I was already a bit tired and it kept me up! One key point of the book is the meticulous accuracy of all the mentioned details, such as the history of the RAF, the Amish, the history of Heidelberg and of course the details about the German police work. I think that is something readers value a lot! I did it - and now I actually learned something about the history of my Handschuhsheim neighborhood. Haha.
The story itself is intriguing and just wonderful. I think that you were able to give all the characters in the book their individual personality - I believe this makes your book differed from many books out there.
It is a great book!!!