There's a convention, when you sell a book, that it's a complete story. An implicit contract, if you like. If you sold a book that was half a book - just stopped mid-sentence and you had to buy a different book to finish the story - most people would consider that a dick move. Likewise a book that's advertised as part of a series can be understood to come with an implicit promise that the rest of the series will exist.

Why do I feel he owes me anything at all? Well, I did pay him for it. If I bought some software and the author refused to fix bugs, I'd feel cheated. An unfinished story is somewhat like that.