my advice is to skip to Demon Night* (and catch up on everything that happened in Demon Angel and Demon Moon here). There’s a couple of reasons for this, but the big ones are:
1) My pacing improved and the writing became more straightforward, IMO. I wrote Demon Night after receiving a lot of feedback on Demon Angel — and many readers who enjoyed that book still had some trouble following exactly what was going on. So I wrote Demon Night with that in mind.
2) Charlie, the heroine, is a newbie to the Guardian world. I know the worldbuilding can be complicated and dense, but Charlie’s outsider status means that everything is explained to her (and the reader) as the book goes along. Now, there’s still a LOT happening and a lot to take in, but compared to Demon Angel/Moon, I think the presentation of that information is easier to swallow, and there’s not as much reading-between-the-lines.
3) Demon Night opens up the second story arc in the series and follows a minor arc resolution in Demon Moon, so it’s a good jumping-on point.
*Don’t get me wrong, I love Demon Angel and Demon Moon. But I know some readers have had trouble getting into the series because of the writing in those books, and recent comments in various locations have made me realize that maybe it’s just easier to skip those two for now (and maybe they’ll be easier to take in after the reader already has some familiarity with the series and the characters).