There are many novels that have reached the status of instant classic, a book like Infinite Jest comes to mind. However, this node is not meant to talk about actual instant classic novels, but rather to discuss the term itself.
Instant classic novel is a term that is a double oxymoron, that is, a phrase that contradicts itself twice. Classic is something that is long established and approved. Novel comes from the word new, because it is a form of literature that originally sprang out of vernacular tales, as opposed to the classic poetry that people had been reading for many centuries. And the word instant, of course, means very quickly. So the phrase "instant classic novel" means something new that is old at the time it is released. In other words, "instant" and "novel" both contradict "classic".
I can only think of one triple oxymoron, which is permanent guest host.