**Answer:** He eats through 8,000 pages.

Remember, when the encyclopaedia is in the correct order on the bookshelf, the first volume is on the left, so the first page is not on the extreme left, nor extreme right. And also remember that he's eating from the first page to the last in a straight line, so doesn't eat either of these 2 pages.

The answer will also remain the same if you consider each sheet of paper to be two pages, he will eat 4,000 sheets of paper, which is still 8,000 pages.