In A Brief History of Time Stephen Hawking investigated the mysteries of where the universe came from and where it is going.  In THE GRAND DESIGN (Bantam, $28) he and co-author Leonard Mlodinow go farther and enter territory where science usually fears to tread.  Why does the universe work the way it does?  Why does it exist at all? Why do we exist? Despite the book’s initial claim that philosophy is dead, the questions it seeks to answer are philosophical ones; they are simply approached from a scientific perspective.  The book begins with a whirlwind tour of the history of science; then discusses quantum physics, the multiverse, and M-theory; and along the way questions free will, a creator, and the very concept of reality. Hawking’s well-chosen examples and endearing humor make even the most difficult concepts engaging and clear.  He is the consummate physics teacher you never had.