There may be some artistic license in the film, but it is not 'fictional'. And the book certainly is not. You can learn a lot of history watching the film.
Cornelius Ryan based his books on interviews with the principals involved, as well as the common soldier. Since he did his research in the early 1960s, most of the generals were still alive to be interviewed. That’s what made his books such popular movies.
For “The Last Battle,” he was allowed by the Soviets to interview Marshals Konev and Chuikov, as well as some regular front line soldiers. He was not granted access to Zhukov.