Depends on the 8-year-old. I found my own books as a kid by haunting the school library. Nobody was there to tell me something was "too old" for me, or that I might get "wrong" ideas from it. My librarian or parents never looked at a book I took out from the library and said "you shouldn't read that."

If your kid likes to read, take him to a library or bookstore and let him browse. You can help him find specific sections.

A few favorites from my childhood (some still sitting on my shelf):

The Mushroom Planet books by Eleanor Cameron.

"Spaceship Under the Apple Tree" series by Louis Slobodkin.

"The Enormous Egg" by Oliver Butterworth.

Asimov, Heinlein, Clarke, Bradbury. Heinlein's "The Star Beast" is great for kids. Classic SF by Wells and Verne. If something was too complex or boring for me, generally I just skimmed over it to see where the story went. I found out on later re-readings that I missed quite a bit from my childhood favorites.

Of course, I read a lot of fantasy as a kid, too, but that's another story.

My own published book (Castle Falcon) is quite long, and geared towards older kids, but your mileage may vary. My second book "Roger Mantis" would be more fun for an eight-year-old but it's still unpublished.