Always be willing to move up or down a level.
Some books are simpler in content but more difficult in language, and a few are just the reverse.
The parent can explain here and there as he reads. The child will be happy to reread this book himself when he gets a little older. You will notice that lots of books fall into two reading levels, such as the Paddington books, which are designated as primary and intermediate books. A proficient primary reader can read and enjoy these books, as well as younger intermediate readers. These are titles which are good read-aloud books for the very young.