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``The world seems precarious to me now,'' the prodigiously productive Ellison (more than 65 books, including some 1,700 short stories) notes in his introduction to this collection of 21 previously uncollected science-fiction/fantasytinged tales, ``everything changes so fast, and no one remembers anything.'' Change is a recurrent element in these typically gruff, exuberant pieces, as is the conviction that humans possess an extraordinary range of talents and powers, few of which we thoroughly exploit. In ``Go Toward the Light,'' for instance, a cynical ``timedrifter,'' who travels back and forward in time as part of a government research project, finds his long-ignored faith stirred by an encounter with a priest in ancient Palestine. Ellison's ability to inject fantastic elements into otherwise grimly realistic situations is powerfully on display in ``Mefisto in Onyx,'' a gritty, terse, deeply disturbing novella about the encounter of a black psychic and a white serial killer on death row. Ellison, 63, needn't worry too much. It's likely that the best of his tales, mingling fantasy and grim, angry realism in a distinctive mix, will be remembered for a long time to come. A varied and powerful collection.