Adult fans of J.K. Rowling can rejoice: She has a new novel coming, for grownups. The author of the mega-selling “Harry Potter” series has an agreement with Little, Brown in the United States and Britain to release her first adult novel, the publishers announced Thursday. The title, release date and details about the book, long rumored, were not announced. Her seventh and final Potter story came out in 2007, and in recent years the British author has said she was working on an adult book and on a Potter encyclopedia. Rowling’s Potter books, which broke sales records around the world, were published by Bloomsbury in Britain and Scholastic in the U.S. Rowling, 46, will now share the same publisher with Stephenie Meyer, whose “Twilight” series at least partially filled the gap opened by the conclusion of the Potter stories. Any Rowling book would seem a guaranteed million seller, although it remains to be seen whether her new novel will have the same mass appeal as Potter. Adult authors from E.B. White to Sherman Alexie have nicely managed the transition to writing for young people, but once a writer is defined as a children’s author, the transition can be tricky. Winnie the Pooh creator A.A. Milne, a successful playwright in his early years, once confessed that he was forced to say “goodbye to all that” after his beloved books about the bear and friends. Margaret Wise Brown, author of the classic “Goodnight Moon,” tried for years to write stories for The New Yorker. Daniel Handler of Lemony Snicket fame and Ann Brashares, author of the “Sisterhood of the Traveling Pants” novels, are among the recent popular writers for children who have had limited success as adult writers.