First the author of that quotation makes an assumption, or an implicit proposition: celebrity is bad. We can agree or disagree with that idea, but for the passage to make sense, we must understand that the author believes this.
They have been admitted to the canon
Canon has a variety of related meanings. The one being used here is
3.c. a sanctioned or accepted group or body of related works
In other words, Twain and Hemingway (or really, the works of Twain and Hemingway) are now included in the collection of books that people generally consider to be important, or great literature.
despite the off-putting aroma of publicity that surrounds them
As mentioned above, the author believes that celebrity is bad. Despite being afflicted with celebrity, the authors (really, their books) have been canonized.