David Ungerer, president of Reston Publishing Co., a wholly owned computer book and software subsidiary of textbook giant Prentice-Hall, has resigned.

Lloyd Rich, formerly director of development and corporate planning for Prentice-Hall, is serving as acting chief executive officer of the local publishing company, said Joseph Kelly, Prentice-Hall vice president for human resources. A search for a permanent replacement for Ungerer is under way, Kelly said.

Prentice-Hall would not comment on why Ungerer, who was instrumental in founding Reston Publishing, resigned or what his plans are. Ungerer, who had been with the company for more than a decade, could not be reached for comment.

Reston Publishing had grown into one of the country's largest and most profitable publishers of computer books. Last year, computer books generated more than $12 million of Reston's sales, and the company was perceived as a major player in the fledgling software market.

Industry sources speculate Ungerer's resignation may be related to a slowdown this year in the booming market for computer books. Net earnings for Prentice-Hall for this year's second quarter, ending June 30, were far below expectations, analysts said.

"There was a high return of computer books noted by Prentice-Hall in its second quarter of this year," one analyst said. "Net per-share earnings in the first half were about 15 cents below expectations."

Industry analysts attributed the slump in the computer books market to "an oversupply of books and too many titles," together with a shakeout in the computer industry that has seen several companies fold and numerous computer models discontinued, making some books obsolete.

Ungerer's departure comes after Ashton-Tate, the leading publisher of database software, recruited Larry Benincasa and Nikki Hardin, two top Reston executives. The two said they couldn't resist the lure of "significant" stock options and higher pay and a chance at something new with Ashton-Tate.

But David Cole, the Ashton-Tate chief executive who recruited the Reston employes, resigned as chairman of the California company last week.

That leaves Cole's slot open, but sources could not say whether Ungerer may be a candidate for the job. One analyst said Ungerer may be more interested in starting his own venture. "One of the reasons a company like Prentice has these subsidiaries is to give entrepreneurial talents a role . . . Ungerer was an entrepreneurial type," the analyst said.