I read Gunter Grass' Tin Drum a bazillion years ago. It took me 4 years to wade through it and, at the end, I wound up wondering why I had expended the effort. I didn't see the point of the novel and wrote it off as a pretty good waste of time and energy on my part.
That doesn't have anything to do with him being a former nazi, but he isn't much of a writer (IMO), either!!
You actually read it when you could have simply watched the movie? Are you a masochist?