Fljotsdale,
I've read the Fionavar Tapestry trilogy a couple of times. They are some of my all-time favourite fantasy books. I've also read Tigana (by Kaye as well), but I didn't like it quite as much.
Jabidas,
The books are about five people who are transported from this world to Fionavar, "the true world, of which our own is only a shadow". They are caught up in a struggle againt the forces of Dark, led by a renegade god. I know that the plot sounds like so many other fantasy books, but these books are really good, IMO. I can still remember the characters and the world, even though it's been years since I last read the books. A couple of parts actually made me cry, a rarity for fantasy books. Of course, it could just be me.
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Nid yw'r hoff o lyfr yn fyr o gyfaill.