Same temperature as here in NYC. I had the same experience with the TV show as some of you. It's been a while since I read the book, but I liked the book very much and this just brought in so many plots that were in other books, I almost stopped watching. The book is really a police procedural, Wallander always working with another policeman and this interpretation wasn't that at all. I do remember clearly how they figure out who did it and how the killers knew the farmer had the money, and it seemed to me that they took care of that in about one minute in this show at the end. One doesn't get the reasoning of Wallander and his crew or the steps they took. Suddenly Wallander is holding the receipt from the bank withdrawal, and it's over. And the scenes about the immigrants are not from this book. It made me think again about Murder on the Orient Express and the extraneous business of the stoning.
By the way, the Swedish Wallander films I saw, even though they were composites and just based on the novels, were not dark (that is unilluminated), but I remember the first film made of a John Harvey novel, a great mystery writer who is still writing, and most of whose mysteries took place in Nottingham, in the North of England. I saw it in Nottingham, at a mystery conference, and the reaction of the audience was above all, "don't they have street lights in Nottingham, don't they have electricity in the houses?" The film was considered "noir" but that should just be the subject. You don't need the sets to be "noir."
I'll still be watching the coming ones, of course. As for the books, it is best to read them in order, but I don't think it will make a difference for the TV shows. I have gone on for too long, sorry.