"Original" is a worn and worrying word, but I have tried to think of modern works that are one-off models in the same way that, say, "Tristram Shandy" is.
The Third Policeman by Flann O'Brien
A book about a bicycle. Surely one of the great comic novels of the 20th century. It's by an Irishman, of course.
Riddley Walker by Russell Hoban
It is no simple business to invent a world, a history, a language, and if "Riddley Walker" looks, on first glance, like Chaucer in need of spell-check, do not despair. By the second page you will speak the language like a native.
The Loser by Thomas Bernhard
Thomas Bernhard, the Austrian who hates Austria, has a reputation for being somehow "difficult," but although "The Loser" deals with such weighty subjects as art, ambition and the futility of endeavor, and although it is scarred by lung disease and suicide, it made me laugh out loud. Sentence by sentence, the structure of the book is at once a mystery and a marvel. Obsessive, elliptical, loopy and so perfectly splenetic. It is a jewel.
Flaubert's Parrot by Julian Barnes
Barnes has written many wonderful books, but this one is like no other, of his or anybody else's. "Flaubert's Parrot" is new, and always will be.
Sixty Stories by Donald Barthelme
The truth is, I haven't read all 60. But "The Balloon" is in here, and it is so fantastically "made-up" and carries such a heavy payload of joy and wonder. It is always with me, something for me to always stretch toward.