The book Z for Zachariah by Robert C.O’Brien has twice been adapted into films. In 1984, the BBC adapted the book as part of the Play For Today series. The BBC changed the location of the setting to Wales, but otherwise kept the script faithful to the book. As the film was aired on TV and not available to buy, you can watch the full version on YouTube.
The 2015 cinema version is definitely not just like the book. The book is about how Ann reacts to the discovery that there is another survivor. The 2015 film trailer has three characters and introduces a love triangle storyline, which was most definitely not in the original book.