I read The Widow and The Strangers in the House, stand alone novels and they were simply masterful. Simenon, using straightforward prose, pulls you into a world that is permeated with quiet menace. This is superb writing.
In 1966, Simenon won the Grand Master Award from the MWA -- and he was under serious consideration for the Nobel Prize for Literature!
If you're in the mood for menace, and want to be transported to a vividly wrought, atmospheric world that seems somehow apart from our own, I can't recommend him too highly. I can't wait to get started on Maigret!