Sci fi that doesn’t feel like a YA novel

Hi everyone, I am looking for a book for someone else, heavy fantasy and sci fi reader who has recently picked a range of books from shelves they did not enjoy because they felt we're almost YA and too focused on romance.

They have fallen out with the genre for a while, so I'd like to give them a gateway back to something they used to enjoy. They say that they wish they could find something more similar to the "sci fi that used to be everywhere" in the 00ies, similar to Otherland by Tad Williams.