Reviewing the series as a whole here rather than just the third book as it is all fundamentally one story just split into three parts.
In summary it's not bad, but it isn't great either. I'm a big fan of timey-wimey pre-destination shenanigans in general and the Obsidian Heart trilogy is built on this and it's interesting to see such a story with a protagonist who is genre-savvy enough to both discuss and exploit it. The flip side though is that in keeping with that as a central theme the "twists" in the series were insanely easy to guess for anyone familiar with such stories and at points it felt frustrating to wait for the eventual reveal, especially when you can tell the author is dragging out the last few moments before it to try and make it land harder. Sorry mate but I worked *that* out about a book ago, just get on with it.
Which brings me to the main criticism I have of the books - the pacing. Mark Morris clearly has a talent for descriptive prose (the description of Frank's patient despatching of the rat in the trenches is a great example) but there's points in the story where events are unfurling at a rapid pace but the prose bogs the whole thing down in excess verbiage and it makes it feel like wading through treacle. Or descriptions of nightmarish creatures that end up reading like a shopping list rather than pumping the adrenaline.
And the same goes for the broader picture of the plot, the non-linear structure effectively means that the "final" confrontation of the story takes place in the second book rather than the third, which I'm fine with because the time-travel aspect gives plenty of scope for things to happen "after" that. But despite the author's constant reassurances (via the narrator) to us that it's not over etc etc the vast bulk of book 3 is spent backfilling the various stable time loops set up in the previous two. The principal antagonist makes a less than handful of appearances and only one of those even vaguely qualifies as a confrontation, and one of them is an oddly out-of-character exposition dump from him that screams of Morris having realised that he's running out of book and needs to answer the question of who the Dark Man is or he'll get lynched by his editor and readers.
So the end result is that stakes are super-low throughout Book 3, even the horrors of WWI are reduced to our hero marking time until he can tick off the item on his time-traveller's TODO list and just when it looks like things might taking a turn for the interesting with the whole zombie thing the main character pulls a Bill-and-Ted manoeuvre and his future self rocks up to solve it with a metaphorical wave of their hand and that plot thread is just snipped off never to be revisited. But hey at least it let us get back to the really interesting discussion of mud and various odours.
This isn't the only time something interesting gets left to die on the vine - the "apparition" of Lynn that popped up several times during the first two books? It just stops and is never explained or developed further. The creation and fates of the various Wolves of London? Nada. We get a cameo appearance from some of them during the climax of the third book but even the exposition-heavy denouement gives them nothing more than a sentence stating that the Dark Man "made them" and shrug of "don't know what happened to them" from Alex at the end. Dr Bishop? Nothing there either, it's just another item on Future!Alex's TODO list and one that isn't covered.
The same goes for 90% of the heart's powers that we see or are hinted at in the books. Narrator!Alex is never shown learning them or using them beyond the bare minimum - he uses it to time travel and to resurrect(ish) Frank, that's it. Maybe Morris was worried about having it become too much of a trump card for Alex if he just went around wielding the heart to solve all his problem but I still feel a bit short-changed there.
Ok, enough complaining. I'm still giving 3 stars here, so what did I like? I loved the concept, The characters were great: Alex - I loved seeing an everyman thrust way out of his comfort zone with no wise mentor figure to guide him on his first steps into a world of magic and mystery. His interactions with his various elder selves are always good value. At points he's petulant, whiny, and just plain tired, but he's also capable of great compassion, loyalty and selflessness. Clover was great too, and I though Morris did a great job of walking that fine line - keeping her behaviour plausible in the moment but also remaining true to how she would act given her real identity as Future!Kate. Present!Kate too, despite only appearing in relatively few scenes was well done, a child character that felt like a child rather than a miniature adult. For all my grumbling about orphaned plot threads earlier I was a fan of not explaining the full nature and origins of the heart itself, we got some fun to think about teases and that was enough. Some things are truly best left to the reader's imagination.
Am I glad I read (well, listened to via Audible) the Obsidian Heart trilogy? Yes, it was flawed but worthwhile. Would I recommend it? Maybe not, I like intricately constructed webs of time and non-linear cause and effect for their own sake, but I'm a bit of a weirdo. So I'd be wary of recommending this to anyone who didn't share my particular interests.