Reddit reviews In Ghostly Japan (Tuttle Classics)

In the past, ZUN has mentioned Lafcadio Hearn (a.k.a. Yakumo Koizumi) when asked about the relationship between Yakumo Yukari and Maribel Hearn. Hearn was a real person who lived in Japan during the Meiji era and wrote a number of authoritative books on Japanese folklore and culture. A few of his books that you may wish to look into are:

• Kwaidan: Stories and Studies of Strange Things
  
• In Ghostly Japan
  
• Glimpses of Unfamiliar Japan

Shipwrecks, by Akira Yushimura. It's a beautiful, devastating story. I can't recommend it enough.

Rashomon and Seventeen Other Stories by Ryunosuke Akutagawa is very much worth reading. It's a bit of a mixed bag (IMO the author is overly reliant on European and Russian literary flourishes), but the better stories are stunning. Hellscreen is one of the best short stories I've ever read.

The writer who got me interested in Japanese culture was actually a Greek/Irish guy named Lafcadio Hearn. Books of his such as Kwaidan and In Ghostly Japan are really engaging and insightful forays into Japanese culture and cultural history. All his books are worth reading, but my favourites are the two I mentioned.

Seeing as I'm mostly now recommending books that aren't novels (sorry), have you come across Hagakure? If it's predominately Samurai your girlfriend is interested in, she really should read this.

I think I found German translations of all of these except Shipwrecks on Amazon. Perhaps you'll have more luck finding it elsewhere (hopefully I was just undone by only looking on an English website).

What time period? Any particular place in Japan?

Lafcadio Hearn has a lot of books on Japanese folklore, many of them ghost stories and tales of the supernatural... so that might fall under your culture requirement, particularly In Ghostly Japan. He's got other books, though, like An Anthology of His Writings on the Country and Its People. His book list is worth looking through; you might find something that catches your eye, and certainly a lot of what he writes about from the turn of the century has an impact on modern Japan.