Now in its fourth printing, this standard critical anthology dealing with the big three among fantasy writers has been brought up to date through the addition of an Afterword discussing the book Tolkien considered his greatest work, the posthumously published The Sil­marillion.   Shadows of Imagination consists of essays by thirteen scholars who treat seriously the fantasies of C. S. Lewis, J. R. R. Tolkien, and Charles Williams. Lewis, Tolkien, and Williams have made the writing of fantasy a legitimate art. These writers, according to Mark Hillegas, editor of and contributor to this collection, have revived the ancient arts of epic and romance, have returned to the tradition created by the Odyssey, the Divine Comedy, Paradise Lost, and Faust.   Hillegas points out that although they often are compared with science-fiction writers, Lewis, Tolkien, and Williams do not write about science, never glorify the machi≠ instead, they fill a void, satisfy a human longing for a "myth to bring meaning again to the universe and human existence.”