Not all of the books you can read are on the AR scheme (if you click on a book then go to details you can see if it is or not, not all of the quiz numbers are correct but you will find the book if you search for it by Title and Author)

You cannot search for a book by it's ZPD level but you can search by Lexile level. Here is a rough comparison chart to help you find a suitable book.

100 = 1.1   200 = 1.4   300 = 1.7   400 = 2.0    500 = 2.6   600 = 3.0   700 = 4.0   800 = 4.8   900 = 5.6   1000 = 7.0

If you read a book that is not part of AR why don't you write a review of it or make up your own questions for somebody else to take and post them on Seesaw?