Audible had a 3 for 2 on series books, and I used two of them on the next two Nightside books.
Nightingale's Lament was interesting, but more for the odd powers that Green gives some of his characters than for any particular plot reasons.
In Hex and the City, though, he moves the story in a new direction. Green has dropped hints in the first three books about the origin of the nightside and about protagonist John Taylor's mother. I'm used to getting these sorts of dribs and drabs of backstory in other series (Butcher does it a lot in the Dresden Files), and it can be a bit irritating; it feels like a way to stretch out a minor mystery that may never be resolved. But in Hex and the City, Green moves the backstory into center stage, which I think is a bold move.
Unfortunately, the boldness is offset by somewhat shaky execution. The Nightside series in general is patterned like a hard-boiled mystery series. One thing that Green should've absorbed from other books in the genre is that, while there may be many false leads for the detective, each one usually brings him a bit closer to his quarry. But, here, Taylor has a series of confrontations with putative witnesses, and each one ends up saying a variation of "I don't know the answer and I can't even guess. Maybe you should try someone else." Although those confrontations are interesting in their own right, any one could have been left out without changing the flow of the story at all.
Overall, though, Green's resolution of so many issues in one swoop sets the novel above the previous two in the series, and is enough to keep me going through the next one.
Speaking of noir mysteries, Burn is a throw-back to older crime fiction. Unfortunately, author Sean Doolittle doesn't really do anything new or innovative. I wanted to like this, because I kind of liked Rain Dogs. But, just like that book had a by-the-numbers quality to it, so did this one, except that nothing really stood out at all.