When you save your customized book, it is saved to your user profile. You can navigate to your user profile by clicking the menu with your name on it in the top right corner and selecting "Profile."

Once you're in your profile, you can find your custom quizzes and questions by hitting the "Quizzes & Questions" tab, and find your books by hitting the "Books & Concepts" tab. You can also archive your books if you're done with them and want to stay organized; if you archive your books, they will go to the "Archived Books" tab.