From Part One:
There are precedents for writers adopting a pen-name when they try their hand at other genres, but Mr. Black makes it clear that he’s not slumming. His conversion came when he discovered Georges Simenon’s romans durs – the hard novels – which are distinct from the novels that feature Detective Maigret, for which Simenon is best known. Mr. Banville has written eloquently and at length about the staggeringly prolific writer, who wrote 193 novels under his own name and over 200 more under 18 pseudonyms. Banville makes no bones about his devotion: "He’s an extraordinary, extraordinary writer."
Another author Banville cites as an "exemplar" of "existential crime fiction" is the work of Richard Stark, the alter ego of Donald Westlake. Both pen crime novels, but Stark’s novels are more hardboiled. Westlake employed a pseudonym to convey a mode of storytelling distinct from its predecessor so as not to confuse or disappoint his fans.
Part Two here.