Please don't bother, they're massively overrated. There's much better "magical boarding school" fantasy out there. The world-building and writing in HP are really shoddy. A lot of the characters represent unexamined stereotypes about Jewish people, POC, and fat people. Not to mention the author is a virulent transphobe who is using her wealth and power to actively lobby for harmful policies. If you missed the Harry Potter nostalgia train, don't buy a ticket now.

I think they meant they felt hollow when the book was over and they had to leave its world and characters behind, so more like leaving the book made them feel that way, not the book itself. So the problem now is that they're not emotionally invested enough in what they read to care when it's over.

I like Oryx and Crake, just re-read it myself -- but I also really enjoyed One Hundred Years of Solitude in high school. I really enjoyed the magical realism, and the fact that it's an epic family saga covering several generations. Be warned though, at least one character marries a child, which I found weird even at the time and looking back as an adult, was super gross. However I would still read the book again. It's a deep world to get lost in.

Adaptation, by Malinda Lo. Has a sequel called Inheritance. It's a really interesting approach to first contact, and it doesn't just devolve into a war. You don't see or even know about the aliens for a while though, it's more of a mystery at first where it's not clear what's going on. It's YA, with a bit of a love triangle element.

Have you read A Tree Grows in Brooklyn? It follows a girl from a poor immigrant family from her childhood to her coming of age. Set in Brooklyn in the early 20th century. I loved it growing up -- Francie is a wonderful and well-drawn character.