I encountered the following sentence in a book.
Okay, I guess the meaning is easy enough: I asked him if he would come tonight. This の, however, was a slight surprise. Grammatically speaking, since it comes after 来る, it has to be nominalization of some kind. But I have no idea why か would require any kind of nominalization when it’s used for indirect speech questions.
In fact, if I had to say something like this, I would most likely say:
Or even just:
I thought of three (rather unsatisfying) explanations. First one: の (or のか) is a common interrogation particle in casual speech, so one could say to a close friend: 今晩来るの（か）？ However, I highly doubt that the の in the sentence is this kind of の, since it does not sound like direct speech is quoted (at all).
My second idea would be that this の is an emphatic particle, just like when we end a sentence with のだ・んだ. But why use it here? I have no idea.
The third option is regular nominalization, which is used here for some reason. So the sentence would be (literally): I asked him if the fact that he will come is. But then, why would this structure be used at all?