Birth of the Firebringer by Meredith Ann Pierce
Meredith Ann Pierce is one of my personal favorite authors. This book (actually it is the first in a trilogy) is a story of a unicorn. It has a strong plot and new ideas. I highly recommend it.
Through the Looking Glass by Lewis Carroll
You may or may not like this book (I love it, but it is hard reading for many). There is only one mention of a unicorn, but it is a funny part of the book.
The Last Battle by C.S. Lewis
This is the last book in the "Chronicles of Narnia." The unicorn is a main character, although not exactly my favorite. However, the book is a good one with a lot of underlying Christian symbolism.
The Unicorn and the Lake by Marianne Mayer
The story is wonderful, but the pictures...It is illustrated by Michael Hague. If you are familiar with Hague's earlier stuff, you may not like the illustrations.
The Unicorn Alphabet by Marianne Mayer
This is also illustrated by Michael Hague, but has better pictures than The Unicorn and the Lake. It is an illustrated alphabet of medieval objects, etc.
Many Waters by Madeleine L'Engle
The unicorns are not a main part of this book, and like all L'Engle books, it is science-fiction. Everything is very logical, but there is human interest added, too. This is a very Christian book.
A Swiftly Tilting Planet by Madeleine L'Engle
This book is also sci-fi, with Christian themes. The description of the unicorn is not my ideal; the unicorn has wings.
Here There Be Unicorns by Jane Yolen
This is an excellent compilation of poems and short stories done by the author with spectacular illustrations.
The Last Unicorn by Peter S. Beagle
This is a beautiful book with the main character a unicorn. Beagle does amazing things with words.
The Unicorn Sonata by Peter S. Beagle
Another masterpiece of Unicorn. This is my favorite unicorn piece of fiction. I highly recommend it!
The Unicorn Tapestries by Margaret B. Freeman
This is a nonfiction work detailing the Unicorn Tapestries, and the medieval/rennaissance unicorn in general. It has lovely photographs and medieval art, as well as much information.