The Gore Vidal book is "Julian". Check it out (library pun!).
John's assessment of "The Harp of the Grey Rose" is correct. But that was straight swords-and-dragons, whereas De Lint's usual genre is "urban fantasy". He skillfully blends Native American and Celtic myth and magic in urban settings such as Ottawa (his early novels) or a synthesized larger city that I've always assumed resembled an alternate Toronto.
His lead characters are mostly female, artsy-punky-spunky streetwise free spirits who sometimes get a bit tedious when the character type is repeated from story to story or novel to novel. However, his better work, like the story collection "Dreams Underfoot", is definitely worth checking out.
In addition to being a writer, De Lint is a Celtic musician and that aspect shows up in his novels and stories a lot as well.