As I find time I am trying to read both from the letters and from the Introduction. This volume of the Letters might be the best available, but I find the introduction to be terrible. The writer goes on at much excessive length about his own views of philosophy and the world, and it becomes unclear whether he is speaking for himself or is accurately reporting on Ninon. I find writing like this to be totally unacceptable because I cannot trust anything that is said - never knowing if it is Ninon who said it, or this biographer who fancies that we care about his own opinion.
I found these other links to English books on Ninon, but it looks like some of the best original material as to Ninon's views on philosophy remains only in French.
Memoirs 1 (Griffin - part spurious) https://archive.org/details/memoirsninondel01svgoog
Memoirs 2 (Griffin - part spurious) https://archive.org/details/memoirsninondel00svgoog
NDLE and Her Century (Roswell) https://archive.org/details/cu31924028180101/page/n13