Quick linguistics lesson.
Tolkien had Opinions, with a capital O, about Elves and adjectives. Pretty sure he gave his editors heartburn expressing them.
Here's the short version. Elvish = pertaining to the languages of the Elves. Elven = pertaining to everything else having to do with Elves.
Example: The Elvish tongue. Elven robes. Elvish writing. Elven settlements.
He didn't like elfish or elfin at all, pretty much.
"Elvish, elvish has been changed to Elfish, elfish 7 times but left unchanged 3 times. I view this procedure with dudgeon. The older and 'historical' form elvish is still recognized, and appears even in such popular dictionaries as the 'Pocket Oxford'. I suppose I should be grateful that Cox and Wyman have not inflicted the change from elven to elfin ..." - Letters #236
And there's your daily proof that Jason Todd has, in fact, read a book. Enjoy.