The Thirteenth Unicorn is definitely written with a tween/ early teen age group in mind, which I already knew going in and so wasn't disappointed with the overall book. It does remind me a lot of the Narnia series; portal to another world, two boys and two girls, having to beat an evil witch etc, but it has enough differences (the inclusion of other fantasy elements) to keep it distinct.
I am an avid fantasy reader/ writer and look to read books that can keep the audience engaged. The author's style is easy to read and he keeps the story flowing at a fast clip; necessary for a good fantasy. I enjoyed reading it.
My only issues with the story is how fast the pivotal scenes seem to happen, without any fleshing out; battles are over in a paragraph, vital 'weapons' are retrieved without incident. It seems that these are events that could definitely benefit from a little more description. This is the only reason I felt let down.
Overall, the descriptions in the book are well thought out and give a pretty good image of the surroundings; if the author could add these kinds of decriptions to the main turning points (which are what makes a good story) in the book, it would be even better. I could see this book re-written for a more mature audience, where the author could delve into more detail about the history of Camelot, and the other worlds and races.Necromancer (The Dark Rising)