Tricky Data of Bookworm's Movement

What was the data given?

Yes, bookworm only moved 10 mm distance. Read the given data given once again. 

Books are arranged in the way we normally place on the shelf. Normally, they are placed in such a way that the their spines (where most of books have name) are easily visible to us. 

If A, B and C are books having 10 pages each then with the normal placement the pages would count from 10 to 1 if counted from left to right. 

In other arrangement, they would count from 1 to 10 if counted from left to right.

See below.

Now, as per data given, the books are arranged in normal way on the shelf and the bookworm started from first page of first book to the last page of last book.
That is it must have ate only middle book which is 10 mm wide.

Hence, the bookworm must have moved only 10 mm.