The Last Unicorn was first published in 1968 and since that time has been published around the world in many different languages and editons. It tells the story of a unicorn who discovers that she may be the last of her kind. She finds it impossible to believe because there have always been unicorns. Unable to stop thinking about the possiblity, she sets off on a magical journey to find out if she truly is the last unicorn. Along the way she meets many memorable secondary characters (some friends, some enemies) and becomes the only unicorn to ever know what regret (and love) is.
In 1982, an animated movie version of the book was released. Peter wrote the screenplay for the movie which made it possible for the movie to stay true to his original vision of the tale. The movie's plot is almost identical to the original story line of the book. Many famous actors and actresses lended their voices to the characters. Jeff Bridges voiced Prince Lir, Alan Arkin voiced Schmedrick, Mia Farrow voiced Unicorn/Amathea, and Angela Landsbury voiced Mommy Fortuna. 
All of the songs in the movie were performed by America. In March 2004, a dvd version was finally released, but the video quality and sound was not very good. Luckily, in February 2007, a 25th anniversary edition dvd was released in widescreen with digitally remastered audio and video. It also has a beautiful foil-printed slipcase. You can purchase copies of this dvd autographed in three places (the dvd, the dvd case, and the slip case) by Peter S. Beagle at Conlan Press (only copies purchased through Conlan Press help Peter earn money!).
In February 2007, Subterranean Press released The Last Unicorn: The Lost Version in a cloth bound hardcover signed limited editon. It is now out of print (you may be able to find a copy on ebay or at a book store if you're lucky). It is the 80 page draft Peter submitted of The Last Unicorn. It is similar in some ways to the work we all love, but also has several differences. Besides the 80 page fragment, the book contains an introduction by him explaining what he was thinking as he began to write and invision the story and an afterward in which he explains the conections between this first version and the finished one that was eventually published. It is a very fascinating read for fans of The Last Unicorn, especially since it gives us a look into what Peter was thinking when he came up with the story. The 80 page work, along with several other never before published short stories by Peter, will also be in The First Last Unicorn & Other Beginnings which is supposed to be published in 2007 at some point (as of June 2007, no specific release date has been announced). However, I do not believe it will contain the introduction and afterward that is in the Lost Edition.
Peter had always said he would never write a sequel to The Last Unicorn. In 2004, while plans were underway to produce an audio book of The Last Unicorn, Connor (someone who works at the publishing company or his agent, I'm not quite sure which is the most accurate way to describe him) wanted to write on the postcard advertising the audio book that it would come with a new never before published story set in the same world. Peter refused at first, but was begrudingly talked into it because he was told it did not have to be a sequal, but could be an unrelated story in the same world as The Last Unicorn. Full information on how the story came to be (I just paraphrased) can be found on the Conlan Press website.
What resulted was Two Hearts a heartbreaking (I cried at serveral points while reading it) and beautiful follow-up to The Last Unicorn that won a 2006 Hugo Award for Best Novelette. It features all of the same characters we loved in the original story and introduces a new character, a young girl named Sooz. It is available for free from Conlan Press when you order The Last Unicorn audio book. It will be published in a beautiful signed hardcovered edition, limited to 3000. It is also available in The Line Between, a short story collection of Peter's, has been published in several volumes of Best of Fantasy 2006 collections, and was the cover story in the October/November 2005 issue of Fantasy & Science Fiction magazine. I recommend you order one of the audio books to get the special limited edition though, like I did! (Not only will you get the gorgeous limited edition version which is a great value, but you get to hear The Last Unicorn read by Peter S. Beagle!). At some point in the future, Peter will be writing (is already writing?) a book about Sooz's story, which would be a full book length sequal to The Last Unicorn.