I want to write a chapter that would benefit a sub-plot I have in the book I'm writing,but doesn't have any of the major main plot points in it. Obviously I have them all coming together towards the end, I was just wondering if it would be risky writing another chapter just to contribute to a sub-plot?
Speaking as a reader, I wouldn't mind reading one in a book, so I'd say go for it! Sub-plot chapters can actually come in nicely as a sort of break from the main plot, even, if used in the right places. Just keep in mind to treat it as a proper chapter and not a filler one; be sure to include something that gives more flesh to the characters and contribute to the overall story itself (as each sub-plot should anyway), and pace it nicely, and all should be great.
I'd suggest not having too many of these chapters, though. If a sub-plot calls for many chapters dealing with it, one should probably consider dropping the "sub" from it c: