First, this was WAY too short to have counted as a book. Novelette, maybe. But the whole thing was rushed - in two days time, they're in love and engaged, having done the deed multiple times. THAT fast.
Second, the heroine is like twelve-year old. She has no grasp on her life, no idea what people are like, no sense of maturity about her at all. It was kind of wrong, how weak and feeble of character/mind she was.
Third, the author doesn't research. Stitches aren't sewn in one at a time, for example. When you write a vet into your story, you kind of need to know these things.
Fourth, we're *hinted* at his background, but nothing is ever explained. Was he centaur because his mother was human at the time of his birth, or because his father was human and had the dominant genes, or what?
Finally, I... have centaur issues. Anatomical issues. They're my personal issues, I know, but... two separate and competing digestive systems, at the same time? I keep hoping to find a centaur writer will come along and NOT make them eleven and half foot tall with two completely autonomous digestive systems. Drives me insane. But nobody actually thinks thru the thing, which... I just was hoping for something *different*, not more of the same. Disappointing.