The topic of how to end a crime/detective novel is truly fascinating, I think. The obvious method is to have the culprit identified and brought to justice, but there are plenty of variations. There is nothing new, for instance, in the idea of a murderer escaping justice. Patricia Highsmith's Tom Ripley repeatedly got away with murder, and earlier, major writers of the Golden Age such as Anthony Berkeley often took the view that the victim was rather more reprehensible than the killer, and that it was perfectly okay for the culprit to evade arrest.
Sometimes, endings can be ambiguous. We may be left in some doubt as to whether the murderer will be caught or not. Or there may be one or two strands of the storyline – perhaps the outcome of a subplot – that are left unresolved. I've tried this a few times myself, occasionally resolving the subplot in a subsequent book in the series. It's not an easy trick to pull off, but when it works, I think it can be just as satisfying as a fully resolved storyline.
I've been tempted to muse on this topic after finishing the latest Wesley Peterson novel by Kate Ellis, The Jackal Man. It's a splendid book, possibly the best that Kate has written, and I certainly recommend it. It features a string of serial killings in the present-day that echo similar crimes committed more than a century ago. One of the connecting links is an obsession with Egyptology and ancient rituals which the Egyptians practised in relation to the dead – quite gory stuff, but handled sensitively.
It is a very well made story indeed, but I was especially fascinated by the final pages, which have about them a pleasing – and chilling – uncertainty. Kate, like me, is keen on the traditions of the genre, but this is an excellent example of how to end a book by leaving an important question unanswered. Of course, I will spoil the story by saying any more, but I'd be interested to hear your thoughts on unusual endings of detective stories.