To pass a book quiz, a student must score a 3/4 or higher (at the default setting), which amounts to a 75% or higher and shows a solid understanding of the reading material and a strong effort on the quiz. To learn how to pass a quiz, see How Quizzes Work and Scoring Rubrics.
To adjust the passing threshold, see Customize Scoring Difficulty.
You can also allow students to revise or retake quizzes. See how here.