That's frankly a bit sensationalist, in my opinion. The guy was a stasi informer? Okay, so were millions of others. Given his status in GDR, it's hardly surprising. It's not as if he was passing himself off as a full-fledged dissident. It may disqualify him from being regarded a hero, but being regarded a national treasure by the Honecker regime pretty much already takes care of that. It's not a good reason to stop reading his books
The author struggles mightily with subtleties well beyond his reach when he discusses possible SS membership. Not surprisingly, given the ambivalent status of Police Regiments that were not raised within the SS framework, but subsequently designated "SS". For that matter, it's a pretty blurred line between the SS and the German police in general. Don't see much point in holding this sort of war service as a young lad against the man, it's not as if he signed up for the rape and plunder.
In short, I would say that for the value of his literature this means nothing, for him considered as a writer it makes him a more complex and hence more interesting figure and for him as a person (which is of no significance to anyone except his friends and family) it makes him less admirable.