I read this recently too and completely agree! I've followed Stephanie Perkins' blog for a while and I'm pretty sure she shared there that she wrote this book having never been to France. (I believe she did go last year for a research trip.) I kept thinking about that the entire time I was reading ANNA because I was continuously marveling at how she made Paris come alive for me. I've never been myself so I would have no clue if she got any details wrong, but it all seemed entirely believable. What's more, Paris is a place that's never been high on my list of places I wanted to visit but after reading this book it's jumped up a few spots.
The setting was so well done, it was actually a little distracting to me reading the book because I kept thinking, "How did she KNOW that?" The writer part of me was constantly wondering about her process--how much of the details and descriptions were added or changed after her trip? It's something I'm especially observant about and attentive to right now because setting plays a big part in my WIP, and it's set in a city I grew up outside of and lived in for 3 years, and yet I still constantly worry I'm not getting things right. It would feel very daunting to pick a city I had never been to.
In addition to loving the setting, I thought the novel had wonderful characters, humor, and the whole premise of going to a boarding school is so fascinating to me (loved FACTS OF LIFE growing up).