Short version: fun in spite of its flaws, well-written and a good choice for someone looking for some feel-good fantasy.
Long version: I didn't learn until after I finished this book that it started published life as serialized kindle singles. Once I did, some of the book's flaws made perfect sense. See, there aren't chapters in this book, there are parts, five of them. While each part isn't exactly an unbroken wall of text, the lack of chapter breaks gave it a kind of meandering, loose, occasionally long-winded quality. In fact, especially in the earlier parts, there would be a whole lot of meandering, and then a brief flurry of action just before the end of the section. Makes a lot more sense knowing the author had to be sure his audience was hooked for the next part. In novel form, it makes things rather jerky. There were also several instances of "if I had only known then," which probably serves the same purpose. When this was turned into a novel, it would have benefitted greatly from some editing and reformatting. I don't know how these things work, though; possibly that wasn't an option.
Obviously I found a lot to like about this book if I'm giving it four stars. The writing and narrative voice were strong and our heroine, Bekah, is instantly likable. Listening to her tell the story made the meandering parts less of a slog than they would be in the hands of a less-capable author. She and her romantic interest actually talk, hang out, bond, go through things together, have common ground, and communicate like actual adult human beings, about things like consent and the rather unique challenges their relationship faces. Bekah actually makes a pretty meta comment about this at one point; like the "if I had only known then"s, there are a few of those sprinkled throughout the story. They could be a bit blunt, but at least our leads were the kind of savvy, snarky people who might reasonably make observations like these.
Though there are a few fairly brief moments of shock and violence, this book overall is pretty fluffy. If you're looking for something weighty and grim, you've come to the wrong magic house. The plot is certainly extant but nothing mind-bending, though I still think the overall quality of the book is pretty special.
While Bekah finds out a lot about herself that she didn't know before, I can't say she or any of the characters grow or develop that much. She never has to make any hard choices, and she sidesteps any potential moral pitfalls by being strong-willed, sensible, and good. There's nothing wrong with this, of course, and I certainly don't think a character has to suffer for them or their story to be worthwhile. Static characters are less interesting, though. A great deal is made about the price of magic but while other characters are shown to have suffered greatly for it, Bekah seems to sacrifice very little. It was a bit convenient and pat.
But there's a place for uncomplicated stories like that, and as far as options go, Heirs of Grace is a pretty good one. Our heroes are likable, the dialogue pops, and the world is vivid and unique. While not quite realizing its full potential, it's still a gem and worth the ride.