Both the original book and the later film were written by
Peter S. Beagle, one of the premier writers of
high fantasy over the past half-century.
Briefly, The Last Unicorn is the charming tale of the Grail Quest of the last free Unicorn to find and rescue the other unicorns. She is accompanied on her journey by Schmendrick, a sad-but-wise luckless magician, and Molly Grue, former sweetheart of a corrupted Robin Hood imitator. En route, she must cope with various individuals with their own agendae, including a fellow magical creature, a merrily self-exploiting witch, and a grim king in the tradition of the Dolorous Knight or of a doomed Fisher King.
Sometimes hilarious, sometimes poignantly astute about people and life in general, The Last Unicorn also has a wealth of symbolism, a fact intensified by the film version. The unicorn, a traditional lunar figure and associated with the medieval Christian tradition, finds herself set against a preternatural bull, a traditional solar figure associated with the religion of Mithras -- with the religion which had been Christianity's most powerful rival many centuries ago. The harpy Celano, who represents lunar eclipses, is another opponent of the lunar unicorn. And so forth.
The book is a masterpiece of wry whimsy, serious insights into the human condition, and high-spirited parody which verges on silliness. Peter S. Beagle's script for the film makes it one of the two most spiritually genuine films I've ever seen - the other is The Neverending Story.