Double Barrel is a mystery, but not much of one. Inspector Van der Valk himself reflects that he's bored by the case, doesn't care about the anonymous letters being sent out which have driven two women in the small Dutch town to commit suicide, and drove another into an insane asylum. Which is a pretty shitty attitude to take, but Van der Valk is full of those.
He's an oddly frustrating main character because every time I start to get on his side, he says, thinks, or does something I find unpleasant. Yes, it's silly of me not to like him because he considers Hemingway an overrated writer. But there's an elderly man, Mr. Besancon, survivor of many years of unpleasant experiences, first with the Nazis, then the Soviets. Van der Valk considers him the only interesting man in town, and keeps coming by to visit and chat. He reflects on how this is an intrusion, but decides he doesn't care if Besancon doesn't like it. Van der Valk figures he's a cop, he can intrude where he likes. Which again, I consider a shitty attitude, that he can simply barge in and disrupt your day because he has a badge.
It isn't much of a mystery. Basically as soon as he reaches the town, you can figure out who the culprit is. Freeling makes it pretty clear what the letter writer would need, and drops copious hints as to who has that thing. If I can figure out your mystery immediately, it's not very tricky. But Freeling's not really interested in the mystery. I think it took the better part of 30 or 40 pages just to get him to tell us what, exactly, Van der Valk was supposed to be investigating.
I think Freeling's more interested in the nature of the Dutch. He portrays them as a very open people, among those they consider fellow travelers. There's little expectation of privacy, and everyone knows what everyone else is up to. It's a curious circumstance, because it doesn't sound like that sort of typical storybook small town, USA, where everyone knows each other and is cordial and friendly. There seems to be plenty of gossip and peering into people's windows, yet no one bothers to draw the curtains. They simply accept it, or maybe they figure closing themselves off would only heighten the scrutiny.
It all sounds appalling to me, but I'm the sort who prefers to pick and choose who I interact with. That's part of why Besancon is considered so curious, he chooses to live behind a high wall, rather than with an open yard, and large, uncovered windows. It's not the only thing that's curious about him, and that bit at the end felt rather out of place. But maybe Freeling was as bored with his main plot as his protagonist.