Martin Springett illustrated the covers of the three books that make up my Fionavar Tapestry trilogy. The book’s author was Guy Gavriel Kay and they feature the best cover artwork I have come across so far. The books themselves, The Summer Tree, The Wandering Fire and The Darkest Road are pretty good but not to the author’s best. You can follow this link to see Martin Springett’s artwork for yourself.
http://www.martinspringett.com/