Faeries (editor; with Martin H. Greenberg and Charles G. Waugh). 1991. The March of the Millennia: A Key to Looking at History (with Frank White). 1991.
What Is an Eclipse? 1991.
Why Do We Have Different Seasons? 1991.
What Is a Shooting Star? 1991.
Why Do Stars Twinkle? 1991.
Christopher Columbus: Navigator to the New World. 1991.
Why Does the Moon Change Shape? 1991.
Henry Hudson: Arctic Explorer and North American Adventurer (with Elizabeth Kaplan). 1991.
Our Angry Earth (with Frederik Pohl). 1991.
Asimov's Chronology of the World. 1991.
Atom: Journey across the Subatomic Cosmos. 1991.
Child of Time (with Robert Silverberg). 1991.
The Positronic Man (with Robert Silverberg). 1992.
The Mammoth Book of Fantastic Science Fiction (editor; with Charles G. Waugh and Martin H. Greenberg). 1992.
Asimov Laughs Again: More Than 700 Favorite Jokes, Limericks and Anecdotes. 1992.
The Ugly Little Boy (with Robert Silverberg). 1992.
Why Does Litter Cause Problems? 1992.
Why Are Whales Vanishing? 1992.
Why Are the Rain Forests Vanishing? 1992.
Why Are Some Beaches Oily? 1992.
What's Happening to the Ozone Layer? 1992.
Why Are Animals Endangered? 1992.
Where Does Garbage Go? 1992.
Why Is the Air Dirty? 1992.
What Causes Acid Rain? 1992.
Is Our Planet Warming Up? 1992.
The Mammoth Book of Modern Science Fiction (editor; with Martin H. Greenberg and Charles G. Waugh). 1993.
Forward the Foundation. 1993.
Why Do People Come in Different Colors? (with Carrie Dierks). 1993.
Why Do Some People Wear Glasses? (with Carrie Dierks). 1993.