Well, the Nebula Awards are prestigious in the world of speculative fiction. In 1965, there were likely many excellent novels vying for the best novel award. It could have been a novel that explored deep space exploration, or perhaps one that delved into complex social and technological issues of the future. Without looking into the official records, we can only speculate on what the best novel might have been. It could have been a work that was a game - changer in terms of its narrative style or the concepts it introduced.
I'm not sure which specific novel won the Nebula for best novel in 1965 off the top of my head. You might need to refer to official Nebula award archives or do some in - depth research on science fiction literature of that era.