To say Dostoevsky or Kafka only wrote one book you'd have to have read only one book by them
I take the point that the same themes and obsessions recur (Ballard said the artist has to have the courage of his own obsessions), but isn't so much of art about the finer details?
Is 'Hamlet' the same as 'King Lear'? Is it the same as 'Othello'? (Much less 'Twelfth Night')
I think it's helpful if not essential as an artist to have a single obsession driving you. Otherwise it's hard to be driven to write a novel.
Also style is a sort of of personality, and everybody only has the one, unique personality.
I think that you're agreeing with what I said earlier re theorists and grand sweeping statements... it sounds much grander to say "Great authors only write one novel" than to say "Great authors often explore the same theme in several books but they do it in a different way so it's not really the same book, oh and occasionally they might explore a different theme so the book where they do that will obviously be different".
But it's always like that, you don't say "I feel that the novel has run out of steam a bit lately and with cinema and tv there is a case to be made that is no longer the most vital form of art for presenting new ideas in narrative form, although come to think of it I did read a pretty good book the other day..." you have to pronounce "The novel my friends, I declare, it is dead!".
Or I guess it went "There is some debate about the meaning of a book - if someone reads it and understands it to mean one thing are they necessarily wrong even if it's not what the author intended? Is the author always right? Can other meanings creep in unintentionally? This is a slightly complex issue which can't be summed up with a single pithy... oh fuck it, hey everyone the author is dead!"
I think of ishiguro, who's written a novel about/from the POV an english butler, a school of cloned children and a robot and yet seems to have written a very similar book each time, the same techniques and themes again and again
Although the Unconsoled is totally different.