Repetition was critical for my early readers. It helped them gain sight words, feel more confident, and get extra practice.
What to do if your child memorizes a book: For us, practice was a balancing act between needing the repetition to gain fluidity and sight words, and knowing that if we read a book too many times, dear child would have it memorized and would no longer be reading. Dear son was especially prone to this – after one reading he would remember large portions of the whole book for several weeks.
Sometimes, I had them read the whole series start to finish, then reread the series while choosing the order in which they read the books. Sometimes, I had them read the same book two or even three times before moving on to the next book. And sometimes, when we moved on to a new series and then I realized that they were struggling, we would back track and reread a series we had just finished.