William Styron and Kurt Vonnegut went to Capitol Hill yesterday just like any two lobbyists looking for a tax break. In this case, the product wasn't oil or automobiles but literary manuscripts.

In a morning reception, the best-selling authors made their pitch to members of Congress. Author Herman Wouk, whose novel "The Winds of War" is about to be televised, showed up a little later in the day to join the effort. Sen. Edward M. Kennedy (D-Mass.) couldn't make the reception but sent over word that he was on their side.

Here's the deal. Styron figures the raw manuscript of "Sophie's Choice" is worth maybe $50,000. But if he sells it on the open market, it will probably end up "in the game room of some millionaire in Dallas." Scholars and so on won't get to see it unless the millionaire turns around and donates it to some library or university.

If the millionaire does that, he gets a nice $50,000 tax deduction.

If Styron gives it to the same library or university, bypassing the millionaire, he gets, under the current tax laws, maybe a $2.75 tax deduction--just the value of the paper and ink.

"If there is value at all in a manuscript, and let us assume there is some poetic value, then they belong in libraries. That's their home," Styron said in a press conference he and Vonnegut held yesterday in the office of Rep. Thomas J. Downey (D-N.Y.), chairman of the Congressional Arts Caucus.

Downey plans to introduce legislation Monday to change America's tax laws so creators of valuable musical and literary manuscripts, personal papers, photographs and works of art--including paintings and sculptures--will receive market-value tax deductions for giving them to libraries, museums, archives and public institutions.

Donations of papers by public officials would not be deductible. Outrage at such deductions prompted Congress to change the tax laws in 1969 to exclude them, but in doing so Congress also affected artists, writers and others in ways that weren't foreseen.

The results were dramatic. The change "had a devastating effect on the ability of libraries and museums to receive and preserve important collections for future generations," Librarian of Congress Daniel J. Boorstin has testified to a congressional subcommittee.

Before 1969, the Library of Congress received 200,000 literary manuscripts a year, plus a steady stream of musical manuscripts from 35 living composers. After the change in the law, the flow virtually stopped.

What is happening now, Downey said yesterday, is that manuscripts are "winding up in some Saudi's home . . . I think it'd be nice to have them available for public viewing."

"God knows why they want to do it," Styron said of scholars and others who want to view original manuscripts, "but it does seem to be a busy industry. I get requests quite often from people who want to look at manuscripts."

As it is, Styron since 1969 has deposited his manuscripts, including that of "Sophie's Choice," with the Duke University library. The agreement is that the university will care for the manuscripts, scholars can study them, and when Congress changes the law--if it does--the university will become the owner and Styron will get a nice tax break.

Vonnegut deposited his manuscripts--all of them, he said--with the Library of Congress under a similar agreement. "They asked just when the s--- hit the fan," he said. "They got it all. I may have lost a couple of manuscripts because people didn't value them."