Reddit mentions of Postcolonial Green: Environmental Politics and World Narratives (Under the Sign of Nature)

Postcolonial ecocriticism is definitely a rapidly growing field, and you may find it more fruitful to go that route than ecofeminism, since that link seems to be clearer in Maathai's work. Rob Nixon has done some great work in the field, particularly on Ken Saro-Wiwa's life and work in Nigeria, but there have also been several books published in the last few years about postcolonial environmentalism - this book, for example. It's possible that that stuff might lead you back to her feminism, but it seems like a good avenue to start with since you're not totally clear on that connection yet.

Good luck!