Fall reads to me are atmospheric, mystical, perhaps supernatural - books that make one think of Halloween, colder weather and spooky places. Here is a list of books/series that will definitely create such an ambiance for you:
1. The Cainsville Series by Kelley Armstrong is a five book series set in and around Chicago in modern day. It combines Welsh fae lore with mysticism (omens, visions and other unexplained phenomena). It is extremely atmospheric, fast-paced and addictive. I have a review of the first book here.
2. Uprooted by Naomi Novik is another great fall read. It is based on Slavic fairy tales/mythology and is set in a magical forest. The narrative is slow and very poetic, and the story is gripping and unpredictable. I also really enjoyed the magic in this book. The main character Agnieszka relies on her own intuition rather than formulaic spells. She is deeply attached to her homeland and draws on the power of nature rather than complex incantations.
3. The Wayward Children Series by Seanan McGuire. I just finished reading the first two books that are currently out in this series. If you like stories about children finding doors into other worlds (similar to Narnia and Alice in Wonderland), look no further. Book 2 in particular is very atmospheric, dark and spooky.
4. If you are looking for a more twisted or macabre tale, I would recommend Deathless by Catheryne Vallente. The novel features Koschei the Deathless and Baba Yaga from Russian fairy tales and is set in the Soviet Russia right before and during World War II. There is an endless war between life and death, or good and evil. But the opposites blur and it is hard to distinguish who is truly evil and what is life if there no death.
5. Finally, I would highly recommend Arcadia by Iain Pearce. It is a stand alone novel so you do not need to commit to a long series. It is partially set in Oxford, and a school setting is always great for fall. The narrative switches back and forth between three different worlds/time periods. We start out in 1962 in Oxford where Professor Henry Lytten starts writing a novel about a world, completely different from ours. A few pages after that, the reader realizes that the world he has invented is real. And a few more pages later, we find ourselves in a distant dystopian future.
Happy Fall, everyone!