In the late 1970s and early 1980s, Appel wrote a half-dozen genre novels, but all six went unpublished. He finally scored with Time After Time, published in 1985 by Carroll & Graf. The story follows New School history professor Alex Balfour as he is tossed back and forth between present-day New York City and the Russian Revolution of 1917. While seeking an explanation for his unusual situation, Alex attempts to save Czar Nicholas and his family. In the course of the novel, he encounters Ivan Pavlov, Vladimir Lenin, Leon Trotsky and Grigory Rasputin. Along with favorable reviews, the novel received recognition from the American Library Association as one of the Best Young Adult Novels of the Year. The novel gained more readers in a Dell Laurel Edition with cover art by renowned illustrator Fred Marcellino, and it was reprinted again as a Dell mass-market paperback in 1990.

Time After Time is the first of what became known as the Alex Balfour series, although the author usually refers to it as the "Pastmaster" series. The appearance of real-life historical figures became an expected device in the series. Mark Twain and George Armstrong Custer are featured prominently in Twice Upon a Time (1988), an American Library Association nominee in the Best Young Adult Novels of the Year category. Orson Welles, Rita Hayworth and Franklin D. Roosevelt are characters in Till the End of Time (1990), another ALA nominee. In Time of War (2003) takes place during the American Civil War, and Ambrose Bierce is a major character. Sea of Time, set aboard the Titanic, was written in 1987 but never published.

Appel's work as a photographer is represented by the Kathleen Ewing Gallery.

Fiction

• Twice Upon A Time (Carroll and Graf, 1985)
• Time After Time (Carroll and Graf, 1985)
• Sea of Time (1987, unpublished)
• Till the End of Time (Doubleday, 1990)
• Hellhound (with Craig Roberts) (Avon, 1994)
• In Time of War: An Alex Balfour Novel (Carroll and Graf, 2003)

Non-fiction

• Proust 's Last Beer (illustrations, with Bob Arnebeck) - (Viking Press, 1982)
• From Father to Son - (St. Martin's Press, 1993)
• Thanks, Dad - (St. Martin's Press, 1994, new edition, 1997, new edition 2007)
• Thanks, Mom (with Sherry Conway Appel) - (St. Martin's Press, 1994, new edition, 1997; new edition, 2007)
• Old Dog's Guide for Pups (with Mike Rothmiller) - (St. Martin's Press, 2000)
• Thanks, to My Husband - (St. Martin's Press, 2002)
• Thanks, to My Wife - (St. Martin's Press, 2002)
• My Hero: Military Kids Write About Their Moms and Dads (with Mike Rothmiller) — (St. Martins Press 2008)