A Canadian, from Toronto, Guy Kay worked with Christopher Tolkein editing The Silmarillion, before writing The Fionavar Tapestry - A Trilogy made up of The Summer Tree, The Wandering Fire, and The Darkest Road.
Kay writes truly beautiful prose, his characterisation is exquisite, and all his novels are carefuly researched, and painstakingly detailed. Unfortunately this means that he isn't very prolific, turning out one book every two years or so.
His books are characterised by the fact that the world 'Fionavar', first of all worlds, and the setting for the trilogy, is mentioned in in all his books, as part as the mythology of whatever world they are set in. The name varies subtly, but is recognisable
With each book, Kay seems to move a little further from fantasy, with less 'magic' in each, but this doesn't make them any less spell-binding
Tigana is a standalone story, where two opposing sorcerors must be overcome to free a peninsula from oppression - it's a novel without a real villain, unusual in fantasy, where the motives of all parties are clear and understandable. A Song for Arbonne is based in an alternate history France in the age of Courtly Love and tells an epic tale of war, jealousy, love and reconcilliation. The Lions of Al-Rassan looks at an alternate reconquest of Spain, examining religion and how it can distort or view of others and destroy friendship, and the duology, The Sarantine Mosaic made up of Sailing to Sarantium and Lord of Emperors looks at a tale which largely echoes the Byzantine period under Justinian and Empress Theodora.
Kay's most recent work The Light of the Sun looks North, to the cultures of the Viking, Celt and Anglo-Saxon, again in alternate reality.
Guy has also published a book of poetry entitled Beyond this Dark House | 
