You should probably go in the order in which they were released, as Arcane also mentioned. But otherwise, an ideal order would be as follows:
- Owari no Chronicle
- Owari no Chronicle DC
- Kyoukai Senjou no Horizon
- Kyoukai Senjou no Horizon Kimitoasamade
- Gekitotsu no Hexennacht
- City Series
Note that Owari no Chronicle, despite being several thousand years before Horizon, sets up a lot of the technology and gives a lot of explanations as to why some of the stuff in Horizon is the way it is, so I recommend reading it before that.
And The City Series is sort of it's own entity. It takes place after Horizon (thousands of years...sort of), but was written before both Owari and Horizon, so nothing is "carried forward" in-universe. In addition, it's more of a set of short stories, with each novel in the series taking place in a different part of the world with different characters. Feel free to read whenever.
This should make for an ideal read, according to me because that is the order in which I read them/am reading/will read, and it makes sense. Unfortunately, I couldn't find an official source, so this is based on personal experience.
Also see this, regarding the release dates, though I'm pretty sure you would have already seen it : https://en.m.wikipedia.org/wiki/Minoru_Kawakami
Edit : Regarding Gekitotsu no Hexennacht, I found something that ascertains that it's after horizon and before city series:
Clash of Hexennacht (激突のヘクセンナハト) is a light novel series written by Kawakami Minoru (川上 稔) and illustrated by Satoyasu (さとやす). It began serialization in Dengeki Bunko Magazine while its manga adaptation runs in Dengeki Daioh and is set for its first full novel release by Dengeki Bunko in August 2015. It takes place between Kyoukai Senjou no Horizon and the City Series, two other series by the author.
Also the order which I mentioned above isn't a strict order as such, but I have tried my best to arrange them with respect to the enormous time span in which they are set in. They are not very tightly interconnected, but basically set in the same world, as created by the author. So there isn't any specific order, but the above order is what you should follow, according to me.