Here is what I recommend, keep in mind some of them are for adults, so you will need to check if they are appropriate:
- Junji Ito Frankenstein
- Gankutsuou (The Count of Monte Cristo)
- Legend of the Galactic Heroes
- Junji Ito No Longer Human (18+)
- Lovecraft's Manga Adaptations (Start with At The Mountains of Madness)
- Pride and Prejudice | Jane Austen Manga
- Les Misérables Manga
- The Tale of Genji Manga
- Anne of Green Gables Manga
There is more, just look for novels adapted into manga. If they like the manga they might want to read the original.
Also note that some are for adults, so you would need to check them.