For starters, many books come with recommended reading ages. For books for preschool, you want books that are labeled as ideal for kids between three and five years old.
From there, choose books that have subjects that you and your child are interested in. If your kid is really into princesses and magic, kids fantasy books are a great place to start.
If you want your child to learn more about colors and shapes, you can find interactive books that teach these concepts in an engaging way.
Choosing the best preschool children's books really is a matter of matching interests to their reading level.