(FORTUNE Magazine) – Reference works on CD-ROM are outselling their hefty printed counterparts by an estimated 150,000 to 100,000 units. No wonder: Grolier's 21-volume encyclopedia on disk, for example, weighs 0.6 ounces, vs. 62 pounds in book form. The disk ranks sixth on the current best-seller list of CD-ROM titles (see table). Prices range from $79 to $895 for the Oxford English Dictionary. (A good deal, considering that the paper one is $2,750.) The market for such electronic tomes ''didn't exist five years ago,'' says Larry Shiller, CEO of the Bureau of Electronic Publishing. Bureau Development, the company's publishing arm, has two best-sellers. Shiller's prediction: ''Digital publishing will be a $50 billion industry by the end of the decade.'' The market now is about $300 million. The changing names of Central European countries spells growth for CD-ROM atlases, since customers can buy updates as often as companies can publish them. Lost closer to home? DeLorme Mapping's Street Atlas USA allows viewers to zoom in on any spot in the U.S. The viewer can focus down to one-tenth of a mile. Some disks show more than text and pictures. The National Geographic's mammals encyclopedia, for example, has film clips and sound. -- S.L.