Josh paid for three books with two $20 bill .he received $1in change each book was the same price .how much did each book cost ?

The total payment is:
 P = 2 * ($ 20) = $ 40
 The change is:
 C = 1 $
 The total cost is then:
 CT = P-C = 40 $ -1 $ = 39 $
 "each book was the same price":
 Book price = ($ 39/3) = $ 13
 
 each book cost:
 Book price = 13 $

The shelves will hold 270 books in total