The Harry Potter author JK Rowling and the publisher's dream

JK Rowling is about to venture beyond the boundaries of Hogwarts and into the world of novels for grown-ups – with a new book out later in the year.

The title, subject matter and release date remain a closely guarded secret. The publisher would only say that the novel, which is already written, is aimed at adults.

Rowling said of her first published book in five years: "Although I've enjoyed writing it every bit as much, my next book will be very different to the Harry Potter series."

The seven novels about the teenage wizard have become the best-selling book series of all time, with over 450 million copies sold worldwide. Each of the last four works set records for being the fastest selling books of all time and were made into films that reaped billions of dollars at the box office.

Rowling has turned her back on Bloomsbury, the publisher of the Harry Potter series whose first book, Harry Potter and the Philosopher's Stone, was released 15 years ago. Little, Brown Book Group, owned by Hachette, has acquired the rights to publish Rowling's new novel and the publisher will have the lucrative English language rights in both print and ebooks.

Rowling said: "The freedom to explore new territory is a gift that Harry's success has brought me, and with that new territory it seemed a logical progression to have a new publisher."

Bloomsbury said its relationship with the author was "stronger than ever" and revealed plans to publish illustrated editions of all the works. While Rowling said Bloomsbury had published Harry Potter "so brilliantly" the relationship had, at times been tempestuous. Liz Thomson, founding editor of the industry website Bookbrunch, said: "She probably wants to make a clean break. The relationship between author and publisher was very frayed at various times." She speculated the publishing contract would be "seven figures".

Thomson added: "Some will absolutely love it, although without a doubt the knives will be out as well. It will be interesting to see if she can establish a new readership, and of course many of the original Potter readers have grown up. Yet for some adults she may always remain a children's author."

David Shelley, Rowling's new publisher at Little, Brown, said it was a "personal and professional dream come true" to be working with Rowling. He added: "She is one of the best storytellers in the world."

The news will also come as a massive boon to booksellers, with Waterstones describing it as "electrifying". The bookseller is predicting that Rowling has written a mystery story.