The difference between illustration you would find on a book cover (not by an amateur) and between an oil on canvas painting you would more likely encounter in a gallery?
The illustration is secondary to the content, which is the book's text, whereas with fine art, the concept and content is complete to the work, and any text (like an artist's statement) is secondary.
But then again, fine art can also cross that line. Like, using the Mona Lisa for the cover of the Da Vinci Code by Dan Brown.
The pictures here are links to the Chronicles of Narnia books and Christina's World, by Andrew Wyeth, with just enough information to tell you why I've chosen them here.
