THIS is not your typical collection of essays by an eminent middleaged writer of fiction. You know the sort of book I mean - a graceful miscellany of book reviews, introductions and speeches, all wrapped up and offered to the public less as a book, really, than as a kind of laurel, a tribute to the author's literary importance. The magazine articles collected here do more than stand on their own. They jump up and down, they grab the reader by the shirt-front. We may be living in ''an era when the notion of belles-lettres is profoundly dead,'' as Miss Ozick says in her foreword, but it's thriving in ''Art & Ardor,'' which is by turns quarrelsome, quirky, unfair, funny and brilliant.