TOKYO -- From the long lines of gadget buffs at the electronics stores to the factories increasing production to meet demand, the word is out here that Sony has done it again.

The Japanese electronic giant seems to have a new hit on its hands in the form of the "Data Discman," a three-pound, hand-held device that is, in essence, a paperless book.

The small black box is selling faster than any new Sony Corp. product since the now-legendary Walkman came out in 1979, company officials say. Production has been doubled twice since the Discman hit the market in July, and factories are now gearing up for introduction next summer of a U.S. version, although a Sony spokesman in the United States said he knew of no firm plans to begin U.S. marketing.

Somewhat similar to a palm-sized computer, with a screen a little smaller than a business card and a miniature keyboard, the new gadget displays pages of books on its screen, flipping to new pages in response to the user's typed commands. The Discman costs about $350; books, in the form of 3.5-inch optical disks that are inserted into the display machine, range from $15 to $50.

The Data Discman has created a new category of micro-electronic gadgetry here, but it is evidently one that people have been waiting for. Sony sold more than 30,000 of its electronic book players in the first three months after its release here. The company projects first-year sales of 200,000 book players, a pace that would match the Walkman's introductory year.

"Those numbers are impressive," said Peter Wolf, an analyst with Kidder, Peabody & Co. here. "Normally these things start slowly, but this machine has really taken off. It will start to have a big impact next year, when they start selling overseas, especially if the price comes down."

The market to date for Sony's new approach to the traditional book is "mostly young business people, the kind of people who want access to the kind of information that you usually need some heavy volume to find," said Yoshitaka Ukita, the Sony manager supervising the new product.

Part of the appeal, though, is clearly the novelty of using an invention that Sony is touting as "the future of the book." Japanese "salarymen" -- as white-collar workers are called -- gleefully show off the new electronic tool -- or toy, depending on the perspective -- to their friends in the company cafeterias at lunch and on train rides home. In a thoroughly trend-conscious society, the paperless book has become trendy.

"It took off much faster than we thought," Ukita said. "It's selling even though we don't yet have all the titles available we should have on disk."

There are currently about 25 different books available in disk form for the Sony device. Most are reference works -- dictionaries, directories and collections of proverbs and songs -- that use the built-in searching software as a fast electronic index, steering a reader to the desired information.

One disk, for example, contains five dictionaries -- Japanese, English-Japanese, Japanese-English, foreign words and Chinese characters -- that would total about 2,500 pages in book form. The machine can find any word on the disk in about 25 seconds; that's longer than it would take to look up a word in one dictionary, but faster than digging through five.

Sony is also offering an electronic version of a popular "Home Clinic" volume providing first-aid and medical advice. Asked to suggest treatment for a sprained ankle, the disk took the user through a series of yes-or-no questions -- "Is it throbbing pain?" and "Can you walk on it?" -- and finally produced this conclusion: "Call an ambulance!"

Sony is currently trying to arrange a disk version of the Tokyo telephone book, an obvious application that would reduce this enormous city's six massive volumes of phone listings down to take-anywhere hand-held size. Another likely success is the "Jikokuhyo," a thick monthly travel directory similar to the "Official Airline Guide" in the United States.

"One of our big challenges," said Ukita, "is convincing people that they really can read a book without a cover or pages." Reaching casually into his shirt pocket for a disk, Ukita holds up the thin black square and added, "It's hard for people to see that this little thing in my hand is an unabridged dictionary."

"This new Sony thing is a masterpiece of manufacturing technology," said John Stern, who runs the Tokyo office of the American Electronics Association. "It's an optical disk reading mechanism, a liquid-crystal display, an input device with searching software -- and all in the size of a Big Mac. It's a little slower at searching than I would have thought, but all in all it's a formidable new product.

"They could sell these things by the thousands to American businessmen if they got the right software," Stern added. "If you put the airline guide, or this year's annual reports for the Fortune 500, or some city phone books on these disks, it could be a big success in the U.S., too."

At least two American companies are producing small "electronic books." New Jersey-based Franklin Electronics and SelecTronics Inc. of New York both make "books" in the form of hand-held calculators, with text displayed on the screen. Both companies sell Bibles and dictionaries in electronic form, at prices ranging from $50 to $300.

Franklin is about to launch a joint venture with Seiko, the Japanese watch and instrument firm, to sell its hand-held electronic dictionaries in Japan.

But the Franklin and SelecTronics "books" do not permit users to substitute a new book in the same reader. To have both a Bible and a foreign-language dictionary, you would have to buy two different hardware devices.

Sony, in contrast, is marketing a generic "player" that can display "books" produced in disk form by countless publishers. Sony is spending big to make sure that "electronic books" of the future all comply with its standard. The company has signed up scores of publishers here and made a big push among international publishers for its standard, known as the EB standard, at this year's Frankfurt Book Fair.

To date, disks made to Sony's standard display only text. But pictures and sounds are next. In addition, early buyers have complained that the screen is too hard to see, so increased size and back-lighting are also likely in later versions, Ukita said.