David Kipen is a fan of a new book about two very different critics. “Craig Seligman’s new book about Pauline Kael and Susan Sontag is the sassiest, classiest work of popular criticism since Nick Hornby’s “Songbook.” Kael might have praised it as “fizzy,” Sontag could call it “serious” and neither would be wrong. But “Sontag & Kael: Opposites Attract Me” transcends these catchwords to present a funny, smart diptych of two bookish girls from California who took Manhattan and knocked it sideways.”