Problem with your riddle:
"First page" and "Last page" can have two separate distinctions. Either "Page 1 and Page n" or "the first page encountered and the last page encountered." It would be much less ambiguous if you mentioned how many pages are in each book (this won't effect their width) and then said, "This bookworm chews his way from page 1 of chapter I, to page n of chapter IV."
Of course the above action removes most of the fog around what the question is actually asking, which most riddles basically rely on. That is, questions which have multiple answers that can be given, but only one answer that is accepted for reasons which are biased towards what the riddler wants the answer to be.
You should also mention that the bookworm travels in a straight line, I guess. Although that's kind of assumed. Easily fixed by asking what the shortest distance is that the bookworm must travel.