The file that comes up first in the mobi format (unsurprisingly) is the TOC, you are supposed to follow the links to the parts.
Don't know of any users of E-book readers who actually ever follow links in the TOC, perhaps in part because clicking on the links in the TOCs doesn't even work on many E-book readers. First Gen Kindles "worked" on TOC, more recent generations do not. What I do know many users of E-book readers do do is complain endlessly about the amount of PG legalese they have to scroll through at the start of PG books, and how as far as they are concerned these stuff makes PG books intolerable as far as they are concerned so they like getting books somewhere else like mobileread (which are typically just PG books with the legalese removed) For some people the PG legalese offends their esthetics of the book experience so much that they will not even consider the PG books. (PG txt conventions offend my sense of esthetics, but that is a different topic)