I'm wondering what to (re) read next, as I'm inclined towards a Trek novel showcasing somewhat darker / "heavier" themes, something with a bit of an edge.
Barbara Hambly's Crossroad
fits that bill, being a TOS novel with a somewhat more sober tone than is usually associated with the era and with characters hailing from and shaped by very dark circumstances. I read it twice many years apart, and it's held up well and is very thought-provoking. It's dark and heavy with purpose, not for the chic of it.