Here are two crossover books which can be enjoyed by older youth as well as adults.

Midnight at the Well of Souls
Jack L. Chalker

Do you have books that you read years ago that keep coming up in your mind? The Well World series of five books delighted me when I read them, but I haven’t seen or heard about them in decades. I started to think about asking Jen if she could find me a copy of the first book, but while I was checking to see who the author was, I discovered that Lo! It’s been reprinted! 

This is fine old-fashioned sci-fi. Six people, some good people and some despicable, end up in a world composed of 1506 hexagons, each in a totally different environment. When you first enter this world, you are in a Zone. After a brief orientation you are sent through a gate into one of the hexagons. You are now transformed into the body of the intelligent species adapted to that environment. You might be an ambulatory shrub or live underwater. You might be a centaur or a giant insect or something you don’t have a name for. Your gender may change, or you may not have a gender. You retain your memories and personality… unless the residents of your hexagon have the means and desire to change you.

The six new “entries” are racing across the planet, crossing over sometimes terrifyingly perilous hexagons. They are trying to get to the mythical Midnight at the Well of Souls, whatever that might be. It’s possible that the entire universe will be greatly impacted depending on who arrives first. 

Chalker originally didn’t intend to write a series and Midnight stands on its own. Millions of copies were sold worldwide. Many of the underlying themes, like whether uniformity can lead to true utopia, are still relevant today.

Ender’s Game
Orson Scott Card

This is the first in a trilogy, which also includes Speaker for the Dead and Xenocide.

They are grand old sci-fi, the kind with compelling characters, plausible inventions of future science and technology, suspense, and, most of all, exploration of the meaning of humanity and morality. It begins with three prodigy siblings, brilliant children who are studied and assessed. The first is too ruthless, the second is too compassionate, the third is just right. He is taken off planet to be trained and indoctrinated. He is the last hope against “buggers,” a very different species who have attacked and been repelled but may return.

In the trilogy the implications of inter-species coexistence is played out, with gradually expanding understanding of very different makeups that sentient life can take. When is it OK to destroy an entire population? Which, of course, can apply to intra-human conditions as well.