Ms. Edwards, who, like Ms. Roberts, is published by an imprint of Penguin Group USA, admitted this week to “taking” passages from reference books and magazines after a website about romance lit posted side-by-side comparisons of passages from her book Shadow Bear and an article about black-footed ferrets from Defenders of Wildlife.

Ms. Edwards has said she didn’t realize she was supposed to cite sources, explaining that “When you write historical romances, you’re not asked to do that.”

Though Penguin has defended Ms. Edwards, saying she has “done nothing wrong,” Ms. Roberts was less forgiving in her e-mail to the AP: