Let's start with this bit of advice for aspiring authors - if you're going to borrow several concepts without much change from someone else's story, don't namedrop that other book unless you're doing a parody of some sort.
There's a couple reasons for that. First being it just looks sort of lazy once a reader realizes it. And second being, it ruins some of the impact of your story for people who've already read the other one. And if you're already lifting elements from elsewhere without much change, the last thing you want to do is also sap tension and make plot points obvious.
Case in point - this book. It quotes Peter Beagle's "The Last Unicorn" before we get page 1 into Brooks' story. And to be fair, it's not a wholesale ripoff. But the enigmatic talking cat who never gives a straight answer? Yeah. The idea of a unicorn sort of "capturing" those who get too close? Ohh yeah. How about all the unicorns in the world except for one being restrained somewhere? Or the great flaming beast protecting the guy who controls them? How about themes of identity vs appearance running central?
But Liss, you might say, everyone borrows! And it's pretty generally true, they do. But the problem is, here it borrows to the detriment of "The Black Unicorn."
If you put me in mind of "The Last Unicorn" and then present a magic book with a lot of unicorn pictures in it, I'm going to say "Okay, so the unicorns have been trapped in the book and they need to let them out." and then I'm going to start hating the main character, who takes another 200 pages to reach that conclusion.
If you put me in mind of "The Last Unicorn" and then have a character look different to most other people but the same to himself - especially if you've already shown he's protected by magics - I'm going to question whether the change is actual or illusion. And if I don't figure out it's a delusion of the self immediately, I'll probably get it the moment the first of many characters tells our main character he's doing it to himself.
That being said, would this book have been more enjoyable to me if I hadn't already read "The Last Unicorn?" Certainly, though still not a *good* read. It's a very quick read, even though not a lot actually happens and it feels like we spend most of our time in the least-interesting character in the book. A lot of the secondary characters interested me, and I regretted we didn't get to see more of them. The central magic that sent Ben Holiday on his journey could have been really fascinating if handled in a more subtle manner.
In fact, a lot of this could have been fascinating if handled better. But in the end it felt like when someone has a great idea for a book, and instead of writing the book, they just tell you about it. You can see the promise of the ideas, but you can't let them swallow you up and carry you off. You're always aware someone's telling you a story. And thus it was with this. The lack of real action, the lack of actual proactive stuff from our lead and the obviousness of much of what happened completely prevented my immersion in this book.
Overall, I might recommend this to someone new to fantasy. It's certainly light and quick, and I think if you're not familiar with the genre conventions and forgiving of the carried-along hero, it would probably be entertaining enough for the time spent. But that's about it.