What an amazingly prolific writer! My favourite is Canopus in Argos. She plunged into sci-fi when there were still very few women working in the genre. Although, you could argue that Mary Shelley actually started it off in the English language with Frankenstein; or, The Modern Prometheus. There is a good interview with Lessing in the BBC archives, where she talks about meeting a gang of very defensive all male sci-fi writers in a pub in Oxford. The genre was so frowned upon that she describes them as looking constantly ready to be punched.