The toddler barks - ''Rfff, ruff!'' He turns a page, so Mommy can read. ''Is Spot behind the door?'' she asks. He growls as he opens a door in the book to look for the bear. A gleeful ''No!'' comes out as he closes the door and turns the page. Then he starts to hiss, remembering that there's a snake inside the door of a grandmother clock on the next page. At 16 months, this youngster is happily engaged in a learning experience with a pop-up book.
Is this intricate book just a fragile toy? His mother says no. She's feels her son is learning spatial relations, sequence, and vocabulary, is developing his memory and extending his attention span, as he manipulates these action pages.
And she isn't alone. The public is enthusiastic about a new generation of novelty books - pop-ups, fold-outs, overlays, and other unique formats that are now available because innovations in paper engineering and improved quality control have made the books popular with publishers again for the first time in several decades.
Intervisual Communications Inc., a Los Angeles-based company, spearheaded the current trend. Begun by Waldo Hunt, a collector of antique movable books, the company adapted 19th-century designs to contemporary methods. Works by such Victorian masters as Lothar Meggendorfer and Ernest Nister were among those reproduced. By utilizing and expanding the early methods, paper engineers, authors, and illustrators are now working together to produce new and interesting examples of this old art.
Bookstores now provide entire sections for pop-up books, and they're selling well. The Harvard Coop in Cambridge, Mass., for example, has two action-book sections, one filled with new books, the other with reproductions of earlier books.
Movable books are also offered in museum bookstores and catalogs. The Museum of Fine Arts in Boston, for example, stocks a two-book set of Nutcracker paperbacks. One tells the classic story by 17th-century German writer E.T.A. Hoffmann, which Tchaikovsky used as the basis for his popular ballet; the other provides fold-out scenery and cut-out characters. In Washington, D.C., Children's Theatre (Viking), a reproduction of Franz Vonn's 1878 book, featuring four fold-out theater scenes, is available at the Folger Shakespeare Library's store.
The attraction of these action books isn't just for children. Adults buy such reproductions as The City Park, by Lothar Meggendorfer (Viking) for nostalgic or artistic value. They also collect technically intriguing and witty pop-ups, such as Robot by Jan Pienkowski (Delacorte), which takes the form of a tongue-in-cheek computerized letter home.
''Adults appreciate the complicated work that goes into them,'' says one bookseller. ''Parents and grandparents buy novelty books for younger children as keepsakes or 'treasures,' or for their educational value, or as a bridge between toys and books. When kids come to this department the first thing they head for is the pop-up section.''
Favorites for younger children include fold-up books such as Where's Spot? by Eric Hill (G.P. Putnam's Sons). The Magic Boat by Tom Seidmann-Freud (Greenwillow) is very popular with the younger set. It includes a Punch and Judy play script opposite a puppet theater, whose eight actors slide on and off the stage. The Three Little Pigs (Lothrop, Lee, and Shepard) is a clever accordian book in a paper brick dwelling. As the illustrated pages fold out, the triumphant final scene fills the box.
For older children there are pop-up books based on classic comic book figures and replications of science fiction movies. A challenging mathematics book, Anno's Counting House by Mitsumasa Anno (Philomel) has cut-outs on each page to pique a child's curiosity and enhance his or her understanding of sets and subsets.
Another book employing cut-outs is Bruno Munari's Circus in the Mist (Collins). See-through parchment allows viewers to move through a foggy city with traffic, people, and lights appearing then receding. The circus itself is portrayed in brightly colored cut-outs, which give an illusion of depth to the three rings and their varied activities.
One bookseller comments, ''Pop-up books are great gifts for 10-year-olds, an age that is hard to please.
Jan Pienkowski's Haunted House (E.P. Dutton) is an example of just the kind of book this observer was speaking of. There seems to be no end to the funny details and manipulative opportunities it presents, including a saw that actually sounds like it's cutting wood.
Booksellers have some good advice for parents who feel movable books won't stand up to a child's use:
Check a book - even if it's shrink-wrapped - before purchase to make sure everything works. Then take it home and sit down with your child before letting the child use it on his own. Youngsters are curious about the construction of novelty books. Use this curiosity to teach them how to maneuver each page. Younger children can turn discs and use fold-out or fold-up books. When they understand the function of pulling and controlling tabs, teach them how to handle pop-ups.
One bookseller adds, ''You'll know a good pop-up book; it's like good sculpture. It has eye appeal, emotional and intellectual appeal, and you can't keep your hands off it.''