Yesterday I used the phrase magic prose. This morning I woke up alarmed. Perhaps I should have written magical prose. After failing to find a coherent explanation on which use was proper, I did a Google search. Magic prose returned 10,300,000 results, nearly ten times more than the 1,220,000 results for magical prose. Magic it is.
Fowler’s Modern English Usage provides a fascinating and exasperating look at why always using correct English is probably beyond the ability of the average writer. At least those with deadlines to meet.
magic(al), adjectives. See -IC(AL)
Magic tends to lose those adjective uses that cannot be viewed as mere attributive uses of the noun. First, it is very seldom used predicatively; the effect was magical (never magic); the ring must be magical (not magic, though must be a magic one is better than a magical one).
Secondly, the chief non-predicative use is in assigning a thing in the domain of magic ( a magic ring, carpet, spell, crystal; the magic art), or in distinguishing it from others and so helping its identification (magic lantern, square), rather than in giving its characteristics descriptively (with magical speed; what a magical transformation).