For a lot of SFF fans of my approximate age (at least in the US), Norton was one of the authors who first hooked us on the genre. She was prolific, her books were readily available, and she often, though certainly not exclusively, wrote about young protagonists with whom we could identify. Many of her stories were set in a far, far future, where humanity interacted with alien cultures. As well as SF and Fantasy, she wrote a fair amount of hybrid works.
By the time I discovered her books as a teenager, she had already written dozens and continued to be prolific, which meant that between the library, the school library, and the bookstores I had a steady supply of new (to me) Nortons to read. What could be better than that, when you're young and you discover a new favorite writer?
In her later years, some of her inspiration seemed to run dry, and to me it felt like after a certain point nothing was quite as good and definitely not as original. It was as though she was stealing characters, plots, and ideas from her younger self, and reusing them minus that first creative impulse that made them seem special. Eventually, she started collaborating with younger writers, which I am sure gave a big boost to their careers, but those books, too, were generally disappointing. My impression was that the collaborators were probably the ones who did the actual writing with Norton providing the ideas, and that they were trying so hard to write like Andre Norton that they failed to draw on their own strengths without being able to duplicate hers. (In the only one of those collaborations that I remember liking, the other writer was not trying to write like Norton, and stayed true to her own voice and style.)
I think her best stories were written in the late 50's, the 60's, and the early '70's.