There are two words in French that you can use to say better, which are meilleur and mieux. So which one should I use if, for example, I want to say: “the book is better than the movie.” To choose the right one, you first need to know this rule:

mieux means plus bien
meilleur means plus bon

Once you know this, ask yourself what is it that you want to say as in French the way you use the adjective will determine if it should be bien ou bon.

Do you want to say “It is a better book” an “the book is better”?

In the sentence “It is a better book”, better works with a noun. So we should use bon (bien cannot be used with a noun). So you will say:  “C’est un bon livre.” = “C’est un meilleur livre.”

In the sentence “The book is better“, better works with a state verb (be). We should use bon only if it means delicious. If it not related to food, use bien instead of bon: 
“Le livre est bien.”  = “Le livre est mieux.”