Here, Harry Hole, the main character, takes issue with his colleague's grammar. This is not just a quirk of Hole; the other things I'll be blogging about are similar grammatical observations from other characters in the book, so I can only assume Nesbo is the one who is a stickler for precision.
The second two mistakes are factual errors, or judgement differences, so I'll leave those aside. The first, though, is Harry claiming an agreement error. He thinks that the verb should be singular is rather than plural are, to agree with the clausal subject [punks shooting good policemen] rather than the closest noun, the plural policemen. This is a common error; I'm forever correcting it in essays. It's easy to do because there's a suitable noun just before the verb, and our brains have forgotten that the real subject was ages ago, and take the easy option of the closest noun.
He's absolutely right if the subject really is that clause. Why clauses should be singular and not plural, when they don't really seem to be the kinds of things that can be singular and plural, is an interesting question in itself. We might say that it's semantically a singular event, the event of shooting described in the clause. More plausibly, I would say, is that when something can't have number (it's 'underspecified') we default to the singular, which is the unmarked option in English (and in languages generally).
What if, though, the subject is punks? It could be. Then the subject still has a clause, but instead of it being a clausal subject describing an event [punks shooting good policemen], it's a noun punks with a relative clause modifying it: punks [
But here's a thing: we can replace the noun punks with a pronoun, which in this case would be they or them, as it's 3rd person plural, and look! It's totally bad with they no matter what form the verb takes, and it's only good with singular agreement when it's them.
*As far as they shooting good policemen is concernedThe lack of the option of nominative case (they) shows that this isn't a real subject of the gerund (or rather, that the 'subject' of a gerund is not the same as a normal clausal subject). If there was no modifying clause, we could use they, and then we'd have to use plural agreement: As far as they are concerned. So part of this pattern is actually an artefact of the fact that you can't have a modifying relative clause with a pronoun. Note that if we put back in the who are that I showed as understood earlier on, it is obligatorily plural agreement, and it also demonstrates very clearly that punks is outside the relative clause.
*As far as they shooting good policemen are concerned
*As far as them shooting good policemen are concerned
As far as them shooting good policemen is concerned
I really do think it could be either singular or plural, depending on the structure. Not that this matters, much, but it does show firstly that subtle distinctions can illustrate different underlying syntax, and secondly that it's not a good idea to be too nitpicky about grammar in case a linguistics blogger comes along and takes issue with your correction.