I think books can fall in several genres sometimes, though one of them is usually a better fit. When I'm not sure, I pull out DITHOR and read through the "Definition" and the "Common Characteristics" for each at the beginning of the genres.
Historical Fiction: The realistic portrayal of an actual place during a time period in the past.
Characteristics:
1. Mixes true incidents with fictional incidents.
2. Describes settings, experiences, and characters clearly.
3. Has either fictional or true characters that act in a realistic manner.
4. Includes a conflict or problem that is suitable for the historic period.
Realistic Fiction: An invented story that seems real and is set in modern times.
Characteristics:
1. Has characters who are fictional, but act in a realistic manner.
2. Includes colorful descriptions of characters, settings, and experiences.
3. Utilizes dialogue to convey information about the characters and the plot.
4. Involves the main characters in the conflict.
5. Has a rational plot, concluding with a realistic answer to the problem.
If I still can tell after checking out DITHOR, then I just put it where I want it.

HTH!
In Christ,
Julie