Book #4: All about the fae, even called Summer Knight.
Book #14: Apparently all about the fae, and was almost called Winter Knight.
I had a theory about that:
The way you can play on the words of this title make me think that Jim is messing with us/throwing us some info here.
What I mean by that is: When I think of the word Summer I tend to think of as "hot/warm/bright" etc. Yet Night (phonetically) is typically cooler/dark.
On the opposite end of that: Cold Days - Why? Daytime is usually sunny/warmer, so you associate Day with warmth or at least Sunshine. (Yes, these are broad strokes i'm painting here, I realize that during winter daytime can be freaking freezing in most parts of the world.)
I think this is one of Jims' patented "I'm going say one thing that can be interpreted literally (as in, Cold Days = Servitude/Bad times for Dresden) but is also a way of telling us that things are terribly terribly wrong in the background. Summer AND Winter are behaving outside of their patterns, and this is leading up to Big Bad Things.