Maybe my favourite Simenon so far, though partly for personal reasons: I was in La Rochelle recently, though only for about 24 hours, and did a quick Simenon tour of places mentioned in 'Les fantomes du chapelier', also wandering a bit around the harbour area which features centrally in 'Le clan'.
This story is very vividly and plainly narrated, super-easy to follow, and not fundamentally silly (contrasting with 'Les fantomes' in all those respects except the vividness). Places and characters are clearly drawn, and the drama unfolds in clear stages, corresponding to the events of the short period of real history it belongs to. In short, the ideal kind of book for me to work on my French comprehension. But also, maybe, a masterpiece of its type (I hesitate to use such a grand word, but why not, where it's earned?).
I won't summarise the story, as another reviewer here has done that. Since no-one else has yet, I will note that Simenon himself was living near La Rochelle at the beginning of WW2 and was for a while assigned responsibility for dealing with Belgian refugee arrivals, which clearly gave him the material for this yarn.
I feel there is something of Maigret's character in the central figure of Omer Peterson (sp.?). He's not a detective in the slightest, but he has that bulky, taciturn, thoughtful presence.
I'm a little puzzled why it's been issued as part of the 'Folio policier' series, since it's not about crime or cops in any usual sense, though it does deal extensively with rules, authorities, violence and death.