Originally Posted by HarryT
I guess that could indeed count as an "external TOC", but I've never personally come across a MobiPocket book that didn't have a hyperlink to the TOC within the book itself - have you? I agree that one could create such a book, but why would you? Does every device which supports Mobi support Guide items? If not, and you did your TOC that way, you'd have no way to get to it.
I have seen such books. But I would argue that a link inside the book does not change the situation since it points to a new html document that is packed in the Mobi format. So the TOC is not inside the book.