The prime topic in publishing circles this week is: What has possessed William Peter Blatty?

The screenwriter and author of "The Exorcist" and eight other books filed a $6 million lawsuit Friday against The New York Times. The suit claims the paper had hindered sales of his latest novel by negligently leaving it off the Times best-seller list, despite what Blatty says are more than sufficient sales.

"They're selling books like crazy," Blatty said yesterday from Boston, where he is partway through an eight-city tour for the new book. "I contemplated doing this over a month ago because that was the period of heaviest sales. But then I thought I'd just wait and see." What he didn't see was "Legion," his sequel to "The Exorcist," on the Times list.

"It doesn't matter if you're on any other national list," Blatty said. "The only one that influences the bookstores is The New York Times. If you're not on that one, you need a microscope and Sherlock Holmes to find your book."

The suit, filed in Superior Court in Santa Monica, Calif., asks in excess of $1 million in actual damages and $5 million in punitive damages, charging negligent and intentional interference with prospective earnings from the book. A slot on The Times roster "does a lot to enhance sales and promotion," Blatty's attorney, Richard Coleman of the Los Angeles firm of Coleman & Farrell, said yesterday. "And it can influence later paperback editions and movie rights."

In addition, the suit accuses The Times of what Coleman called "intentional negligence," because several weeks ago Julia Knickerbocker, publicity director of Simon & Schuster, Blatty's publisher, had called The Times' Adam Clymer, who oversees the best-seller list, to inquire about "Legion's" prospects--thus ostensibly alerting Clymer to the alleged neglect. And it further charges trade libel because in publishing the list, Coleman contends, "they are, in effect, telling the world that the book is not a best seller, which disparages the product."

"We simply do not comment on any pending litigation involving The Times Company," Elliott Sanger Jr., manager of corporate relations for The Times Co., said yesterday. But "there has been no deviation at any time in our method of compiling our best-seller list--which is a scientific sampling based on actual sales figures. His book has not been treated any differently than any other book."

"Legion," in which an amok demon starts slaughtering Georgetowners, was published by Simon & Schuster June 24 with a first printing of 75,000 copies. A second printing of 25,000 copies was ordered in July, and to date 86,900 copies have been shipped to booksellers, according to Knickerbocker.

While remaining absent from The Times', The Washington Post's and other slates, the book subsequently hit several other best-seller lists, including Time magazine and The Los Angeles Times. And since July 12 it has fluctuated between ninth and 12th place on the tally compiled by Publishers Weekly, the influential trade journal. The discrepancy doesn't surprise veteran list-watchers. The Times and PW lists correspond exactly only about 80 percent of the time, says PW senior editor Daisy Maryles. "For example, we got 'Exocet' a new novel by Jack Higgins three weeks before they did." But then " 'How to Satisfy a Woman Every Time' nonfiction by Naura Hayden has been in the Times for six weeks and hasn't showed up on our list yet."

Crown Books president Robert Haft said of the discrepancy, "I don't regard this as unusual." Although "Legion" has been "selling well" for Crown, "it takes an amount of time to get on the list." With 140 stores across the country, "we see regional variations all the time." Michael Meyer, director of merchandise buying at the 826-store Waldenbooks chain, said yesterday that "Legion" arrived on the company's in-house best-seller list the week of June 25 and dropped off Aug. 13. But Kay Sexton, who oversees best-seller figures for B. Dalton's 680 stores, said the novel never sold strongly for them: "In its first week it was No. 7, but it went downhill from there," ending at No. 17 out of 20 this week.

Moreover, methods of sampling vary from list to list. PW conducts phone polls of 1,600 retail outlets, asks each to name its top five to 15 best-sellers, adjusts the data by sales volume and prints the results. "The PW list," Haft said, "is generally more reflective of small, independent bookstores," whereas The Times' reflects national trends and is "the industry standard." The way that standard is set is another of Blatty's complaints.

Once a month, the paper sends a set of four weekly ballots to some 2,000 independent booksellers, department stores and chains. (Giants like Waldenbooks and B. Dalton simply throw away the paperwork and send in their own lists.) Each week's form already carries the names of 36 top-selling titles from the previous week, and stores are asked to rank each according to sales. But "people are invited at the top of the list to write in other titles," said The Times' Clymer, assistant to the executive editor, and each form contains blank lines for that purpose. The results are weighted by size of store, often checked by phone, and then fed into a computer to determine the volume of weekly sales.

Blatty thinks the blank-line format discriminates against those not among the big 36: "Write-in candidates don't win elections," he said. However, "Legion" only missed one possible month of mailings. "Mr. Blatty's book was in the top 36 in the last week of July," Clymer said, "and thus it went out with the list for August. And he is on our list for September."

In fact, as of Sunday, Sept. 4, "Legion" will finally appear on The Times list in the lowest (No. 15) fiction slot. "Too late," Blatty said. Besides, that's not quite the point: "I'm not looking for the money. That's not what I'm after. I want to know how they compile that list--what is so scientific about the way they put their numbers together."

In sum, he said, "I want satisfaction." In fact, over the years he has developed a reputation as an enthusiastic litigant. "It is true," Blatty said, "that I have been in the courts." Among other parties, he sued Warner Bros. over the distribution of monies from the movie version of "The Exorcist" and Bantam Books over the paperback edition (Bantam's all-time best-seller with 11 million copies in print.) Both were settled out of court. "I don't sue frivolously," he said, "and I have never lost a court case."