It's no different from other books. Start on page 1 and proceed by induction from page n to page n+1.
The main hazard is if the book is only ok. If it's great, you treasure having read it. If it sucks you quit fairly early, no prob. If it's just ok, you probably spend a bunch of time reading all the way through and then regret having done so.
Adventure books like Count of Monte Cristo (disclaimer: I haven't read that one) are quicker and easier reads than literary classics, so the length is less of a big deal.