“Deceptively Delicious,” the cookbook by Jessica Seinfeld whose recipes for concealing puréed vegetables in comfort food for children bore such similarities to another cookbook’s that it inspired a lawsuit from that book’s author, shot to the top of the Amazon best-seller list on Wednesday, nine months after it was published.

The sharp rise in sales caught the eyes of both books’ publishers, who traced it to the rerun on Tuesday of an episode of “The Oprah Winfrey Show” that featured Ms. Seinfeld. The appearance also subsequently lifted sales of “The Sneaky Chef,” by Missy Chase Lapine, the author who is suing Ms. Seinfeld.

The books, similar in theme, content and appearance, remain inextricably tied to each other. On Amazon, shoppers viewing “Deceptively Delicious” are prodded to order “The Sneaky Chef,” and vice versa.

And both books’ newfound popularity came as Ms. Lapine vowed on Friday to press ahead with her lawsuit against Ms. Seinfeld and her husband, Jerry Seinfeld, originally filed in January. The suit charged that the Seinfelds were guilty of copyright infringement and defamation. (It was Mr. Seinfeld who, during an appearance on “Late Show with David Letterman,” before calling Ms. Lapine a “wacko,” mockingly suggested that his wife was accused of “vegetable plagiarism.”)

Armed with a new set of lawyers, Ms. Lapine recently extended her lawsuit against the Seinfelds to include HarperCollins, the publisher of Ms. Seinfeld’s cookbook. Ms. Lapine’s original lawyers left the case because they also represent News Corporation, which owns HarperCollins, Ms. Lapine said.

Ms. Lapine is seeking unspecified damages.

The Seinfelds called Ms. Lapine’s charges “trumped up,” pointing out that sneaking vegetables into children’s foods has been done in cookbooks since the early 1970s. (A lawyer for the Seinfelds did not return calls for comment on Friday.)

In a telephone interview from her literary agent’s office on Friday, Ms. Lapine said she would continue her lawsuit as long as necessary. “I have no expectations or requirements on time,” she said. “I’d love to see truth and justice and fairness prevail.”

Steve Ross, the publisher of Collins, the imprint that published “Deceptively Delicious,” said the inclusion of HarperCollins in the lawsuit did not change its support of Ms. Seinfeld.

“HarperCollins remains thrilled to count Jessica Seinfeld on its roster of talented authors, and continues to stand unequivocally behind her work,” he said.

The “Sneaky Chef” dispute began last summer, when Ms. Lapine received an eight-page promotional brochure for “Deceptively Delicious,” a sort of mini-version of the book. Ms. Lapine said she was stunned to see the similarities between the books, down to Ms. Seinfeld’s cover (a winking chef and an attempt to hide carrots).

Ms. Lapine’s book had been rejected by HarperCollins and was eventually published in April 2007 by Running Press, an imprint of the Perseus Books Group. Six months later Ms. Seinfeld’s book was published.

Each book became a best seller, but Ms. Seinfeld’s celebrity status helped her win a coveted appearance on “The Oprah Winfrey Show,” the ultimate book promotion. As of this week, Ms. Seinfeld’s publisher said 2.4 million copies of “Deceptively Delicious” were in print, while Ms. Lapine’s publisher said more than 200,000 copies of “The Sneaky Chef” were in print.

In March Ms. Lapine published a second “Sneaky Chef” cookbook, directed at women trying to persuade their spouses to eat healthier food. She is currently working on a third cookbook, “Sneaky Chef to the Rescue,” built around specific food-related questions she has received from readers, like cooking for holidays, for dieters and for people with food allergies.

The Perseus Books Group, Ms. Lapine’s publisher, is not a party to the lawsuit, but its chief executive, David Steinberger, has signaled his solidarity with her. “We support our author’s right to take steps to protect her intellectual property and reputation,” Mr. Steinberger said in an e-mail message on Friday.

Thomas Girardi, one of Ms. Lapine’s new lawyers, did not give details on the damages that Ms. Lapine is seeking, but said he expected the lawsuit to stretch into the fall. “This is not something that’s going to be resolved a week from Tuesday,” he said.

Mr. Ross, Ms. Seinfeld’s publisher, said despite the pending lawsuit and the swirl of controversy surrounding her book, HarperCollins has tentatively planned a new book with Ms. Seinfeld, which will be announced sometime this summer.