There have been some amazing entries in the Dark Olympus series, and there have been some duds. I'd say that Midnight Ruin falls somewhere in the middle. In the first book, Orpheus abandons Eurydice to a fate engineered for her by Zeus; she is taken into the "under city" and taken under the protection of Persephone and Hades. In recent entries in the series, she has taken a more active role in Olympian politics. She's also been hanging out with Charon a lot. But, she can't seem to stop thinking about what might have been with Orpheus...
As this series has continued, the less the stories have to do with Greek mythology and the more they lean into Olympian politics. Sometimes this works (I absolutely loved Radiant Sin, which was almost unrecognizable as the Cassandra/Apollo myth) and sometimes it doesn't (Cruel Seduction... what even was that?). At this point, the shift away from Greek mythology means that the names of the characters are really just random names. Orpheus? He's a painter! Eurydice? She was in love with him! If you thought that he was going to cross into the underworld (under city) to get her and be given a task to complete before he could get her out ... you're wrong. He does meet up with her when Charon tells him to, but like I said ... that's about it. Do not go looking for deeper connections because they're just not there.
All that said, I have been eagerly awaiting this entry in the series, because I generally love what Katee Robert does with MMF triads. This one I didn't love, but I can't really blame her for my own preferences. Here, she leans in on the degradation kink in this one, which I was not expecting. Orpheus feels he has to atone for his sin of abandoning Eurydice. He does this by cleaning Charon's kitchen floor, servicing both Charon and Eurydice sexually, and basically being their "good dog." (Sadly, Cerberus is an actual dog-character, because I do think it would have been funny to see a name change or something.) While Robert's work is generally on the kinkier side, this went even further on the spectrum. Just keep that in mind when you pick this one up!
If you're really into the politics of Dark Olympus, I think you'll lean more towards liking this one. If you just wanted MMF, Katee Robert-style, make sure you know what exactly you're getting into!
This objective review is based on a complimentary copy of the novel.