HARRY N. ABRAMS has its first national best seller in Gnomes, by Dr. Wil Huygen, illustrated by Rien Poortvliet.There are 100,000 in the stores now, or tucked under Christmas trees, or living under bridges or mushrooms. Fifty thousand more will be coming into print even as you read this, and 100,000 additional copies are being planned. That's a lot of little people, who are reputed to live 400 years.

Published on Halloween, Gnomes have been taking over America, and even their representative, Ian Ballantine, a man of gnomish aspect and temperament who bought the reprint rights to the book for his Peacock Press, made a bushy-browed appearance on the Today show. But are gnomes real? In other words, do you report this book on the fiction or non-fiction best-seller list? Denver and San Francisco entered the work on the fiction best list; New York said nonfiction, and Publishers Weekly, ever alert called Abrams to check. "Nonfiction," is, the answer. See, if you believe in them, it's real, right? And if you don't . . . call it parody, and spoofs are, to Abrams's mind and ours, not fiction. "And it couldn't happen to a nicer book," adds Abrams's Carol Schneider.