How about instead of asking a whole bunch of adults who know little about the reading habits of 8 year olds, you go ask the school librarian.
I work in a school library and we have all our books with coloured dots on them to help the kids know what book is suitable for them (eg. Yellow dots is suitable for 10 year olds, though younger kids can read them if they can prove it's not too hard).
To check if a book is too hard for a child, we get them to read a page, if they make more than 5 mistakes, then it's too hard and they should go get an easier book.