What is a critical analysis of The Tin Drum by Gunter Grass?

A question that frequently comes up in literary analysis is "what is the relationship between an author's work and his life?" This question came to the forefront in Grass's life when, in 2006, at age 79, he revealed he had been a member of the Waffen-SS.
The Waffen-SS was the group largely responsible for enacting, under Nazi orders, the Jewish holocaust and the ethnic cleansing of Eastern European countries held to contain "subhuman" groups such as Poles or Roma. It was declared a criminal organization after World War II. Grass was drafted involuntarily into the force at age 17 and claimed he was involved in no criminal activity. However, the revelation sent shocks waves through Germany, as well as the literary community. People were especially startled because Grass had been a voice of conscience, exhorting Germans to come clean about their involvement with Nazism.
How does this influence our reading of The Tin Drum? Do we reject a novel that has achieved critical acclaim because its author was once a member of a particularly heinous criminal organization? There are no easy answers to this question. Nevertheless, there is little doubt that Grass's reputation has plummeted since the revelation.
According to Ted Gioia, the revelation lessens the difference between Grass and his Tin Drum protagonist Oskar. Is the three-foot height of Oskar, who refuses to grow up, the metaphoric equivalent of Grass himself, still mentally only a child at the end of World War II? Is Oskar's decision to grow after the war, but only into a four-foot hunchback, a metaphor for the way the war crippled Grass?
While reading literature in the context of biography has regained legitimacy since the period of New Criticism (when the focus was completely on analyzing the text), biographical readings must still be done cautiously. Nevertheless, Grass himself brought attention to his story and its relation to his art. There's no denying that the war preoccupied Grass long after it had ended, such as in his opposition to the reunification of Germany. We know too that he did not shy away from constructing parallels between himself and Oskar, who, like Grass, is born in the Free City of Danzig at roughly the same time as Grass, both of them living through World War II. To what extent Grass is telling his own story through a macabre magic realist lens, however, is up to the reader to decide.