trying to find well-written challenging fiction that isn't widely known

[highlandcoo]

I have a very well-read young friend, who lives abroad, and traditionally I always send her books on her birthday.
It's getting harder and harder to find something she hasn't read, but sending her a book token just wouldn't be the same.
She has read many of the classics, as well as a huge amount of contemporary literary fiction. I know ahe has enjoyed Tolstoy, Dostoevsky, Zola, Gunter Grass, Herman Hesse, Murakami, Paul Auster, David Mitchell ...
I wondered about books in translation that she's less likely to have come across. Open to any suggestions though.
TIA

Oh, and not fiction, but a volume called 'Oscar's Books' about the fate of Oscar Wilde's library (sold off through a window in disposal of his house and belongings) is fascinating. The author is very well-read, and has attempted many, but says that he isn't capable of reading even the ones that are known to have belonged to that collection (i.e. many classics and translations).

Thomas Pynchon - all of them are brilliant and challenging, The Crying of Lot 49 is a good place to start
Don Delillo - Libra
Neal Stephenson - Baroque Cycle
Umberto Eco - Foucault's Pendulum
George Perec - Life A User's Manual
Joseph Heller - Something Happened

Steinbeck: east of Eden
Dos passos : USA
Mailer: executioner s song
Atwood: the blind assassin
Balzac: la comedie humaine/ human comedy
Hannah Kent: burial rites
Flaubert: Madame Bovary
Capote: in cold blood

Their eyes were watching god. Zora Neale Hurston

Late to this party but what an amazing set of titles! I’d add We That Are Young by Preti Taneja, a version of Lear set in contemporary Delhi. And anything by Muriel Spark, so ruthlessly and enjoyably unsentimental. And anything on Backlisted, as someone mentioned above, is worth a read. Jane Gardam and Barbara Comyns were my finds of last year.

Geek Love by Katherine Dunn?

The flowers of Hiroshima by Edita Morris

The Royal Game by Stefan Zweig

Both not well known, but definitely literary treasures!
I think the Edita Morris book is out of print, but you should be able to get a good copy for around £10.