Words that were difficult the first few times a book was read become easier each time. The process of sounding out becomes easier and words are stored in the reader's memory for future reading.
My own children had boxes of favorite books for rereading. You can help your child by having books your child likes available for rereading and encouraging your child to read everyday.
The trick to getting children to read books more that once is to have them choose their favorite books. Many books should be made available. If a child doesn't love a book, it won't be one they will want to read again.
Here's a link with advice for finding good books for kids.