What Ke$ha brings to pop music

Though Ke$ha’s first album, Animal, was a glitter-coated gutter dance party, with the brash singer getting sleazy and leading throngs of revelers into the night, no guitars were invited. On her new full-length album, Warrior, rock music scores a place on the guest list: the LP’s first single, “Die Young,” has a strummed guitar front-and-center.