Mystery/thriller about mysterious drugstore room and secretive owner

I forgot title and the author of this book haha. I began reading it then left it on a plane only to be haunted by it ever since. The book is about a mysterious room in a small town drugstore. The drugstore owner is well liked by the locals but has some secrets(abuse, affairs on his wife) His son begins running the store and dating a woman with a grudge toward the drugstore owner. The woman wants to find out whats happening in the mysterious drugstore room and the rest of the secrets