Whenever you open a book that has a "public" quiz created for that book, you will see that quiz. If a book has multiple quizzes made by different teachers, then currently you will see a quiz selected by our shuffle algorithm. Currently, we don't have a way to see all the quizzes for a given book, or to choose which quiz you want to take.
If you have made a quiz for any book, then your students (any profile within the account which created the quiz) will always see your quiz, not the quizzes made by other teachers. But if you have not made a quiz for that book, then you will just see a randomly selected quiz.