A Darkling Plain is the longest of the Hungry City books, by a fairly large margin. But there is, after all, a lot going on here. There's hardly any padding, so it earns it's 500+ pages honestly.
Thinking about the book, I suspect that I wouldn't have liked it so much, or been so glued to it, if I hadn't already been deeply invested in these characters from previous books. Tom and Hester especially, who see their story draw to what is probably its unavoidable close. The ending is fantastic, emotionally satisfying and almost overwhelmingly bittersweet. Wren remained the character I was most likely to sigh at in exasperation, but Theo really gave her a run for the title. Then again, his missteps seem to fit into what I guess is one of Reeve's takeaways for the series: nice guys may not finish last, but they sure can be dumb sometimes.