A Puzzle

There are three volumes of a book on a shelf in order from left to right: Vol. I, Vol. II and Vol. III. Each book is exactly 1 inch thick. A bookworm starts eating through the books. He starts at the outside of the front cover of Vol. I and chews his way all the way to the outside of the back cover of Vol. III. How long is the hole made by the bookworm? (You can assume he chews in a straight line parallel to the shelf, not some crazy loopy hole or anything like that.