Johannes Gutenberg would beg to differ. A publisher is not a requirement for a book any more than a record company is a requirement for a song.
The same is true for books. Anyone can write some texts amounting to 300 pages of characters on pages. That doesn't make it a book, IMHO.
To think that an intermediary is necessary to create a long-form work is demonstrably false cargo cult thinking. Books were printed for centuries before publishing companies came along.