I didn't realize that this was a sequel until I started reading it. I might have tried to re-read the first book before starting this one had I known. It's been over two years since I read Touchstone, and I don't remember it in detail, but I did recognize the characters right away. Even without remembering much, I was still able to follow the story, so it may not be necessary to read the first book first, but I do recommend it. 
 I found this book rather interesting. I don't know much about 1920s Paris but it really makes a great setting for this mystery.