I really enjoy Scalzi, so I was surprised when I didn't know about the Interdependency series. The nice thing was by the time I finished Collapsing Empire the second book in the series was released (The Consuming Fire). I haven't started it yet, but it's next in the queue!
I found the Red Rising books to be stilted, narcissistic and implausibly Manichean, FWIW. I made it through the first one, the second one was just too plagued by characters making implausible choices and the protagonist scoring implausible successes.
- Vengeful and Vicious by VE Schwab - Collapsing Empire by Scalazi (late 2017, but close enough to 2018)
Not released in 2018, but still fun and new to me this year. - The Red Rising series by Pierce Brown