I'm a librarian who loves anime, manga, and reading a wide variety of genres.
Holly is a young girl who is visiting her great-aunt during summer vacation. At some point earlier in the series, I'm guessing she must have gone exploring or something and figured out how to visit the magical world of Pony Island. In this book, she goes to Pony Island to meet Puck, her fairy pony friend. They're having a picnic together when they hear someone crying for help. It turns out it's a unicorn named Willow who's being attacked by several bad fairy ponies. Shadow, the ringleader, is preparing to do a spell that will give him unicorn powers and allow him to take over Pony Island. He stole the first few ingredients from Willow, and now he plans to trick the Unicorn Prince so that he can get the final ingredient.
I was told it wasn't necessary to read these books in any particular order. A bit of searching tells me that this is probably Book 5 in the Fairy Ponies series, although the only thing I felt I was missing out on was how Holly found Pony Island in the first place.
I bought this for my oldest niece, who has watched a show called Mia and Me on Netflix multiple times. Mia and Me includes elves (whose wings make them look more like fairies, but what do I know?), unicorns, dragons, and a winged unicorn named Onchao. The winged “Unicorn Prince” on the cover of this book immediately reminded me on Onchao.
This is definitely aimed at a younger audience than the stuff I normally read, but I always try to read the books I plan to give to my nieces and nephew. Unicorn Prince had exactly the appeal factors I expected: a girl who could travel to a magical fantasy land, fairy ponies (because a plain old pony or even a Pegasus wouldn't be magical enough), and impossibly beautiful unicorns. Although Holly isn't magical herself, her lack of magical ability actually turns out to be beneficial in this particular book.
I didn't notice any problems with the writing, and the story was easy to follow. I didn't always like the illustrations (Puck occasionally looked a bit odd), but they weren't necessarily bad. The text definitely fits my niece's reading level, and I'm crossing my fingers that she'll enjoy the fantasy aspects here the same way she enjoys them in Mia and Me. That said, from an adult perspective, Mia and Me appeals to a broader age range than this. I actually kind of enjoyed that show. Unicorn Prince, on the other hand, felt too simplistic and flat to me. There was no time to get to know the world and the characters at anything but the most basic level. If my niece ends up liking this and I get the other books for her, I'll read them, but otherwise I'm not interested.
(Original review posted on A Library Girl's Familiar Diversions.)