What is the difference between Unnamed vs Untitled. Are they interchangeable? Can 'Unnamed' be applied to a book, composition, or other artistic work?
Google ngram finds nothing for "unnamed book" and "unnamed composition." It's not actually wrong, but at least those terms are hardly, if ever, used.
In the title of a work however you have a lot of freedom, and there's nothing wrong with naming it "Unnamed." in fact it adds character.
"Untitled" is usually used to refer to artistic works that have not been given a title. This is often the case while the project is still in development, and the title hasn't been decided on yet; news reports might say that a director/producer/star is attached to an untitled movie, for instance. But some works of art are never given a title, and "Untitled" might then appear on a placque next to it in a museum or gallery, for instance.
"Unnamed" can also be used to refer to someone who doesn't have a name (such as an abandoned baby who has been found, or a child who hasn't been born and named yet). But more often it's used when the name is known (by someone), but they've chosen for some reason not to reveal it. For instance, accident victims are often unnamed by police until after relatives have been notified.