I've seen how brilliant people on MN have been at finding obscure books before, so let's see if you can find mine. I last read it many years ago so I'm very fuzzy on the detail.
-- I read it in about 2003. It could be much older than that but won't be newer.
-- It tells a generational story after some kind of apocalyptic event. There are several sections with different protagonists and we find out that the later protagonists are descended from the first ones because they discover their diaries/notebooks/some other written material. It's possible that these contradict what the generation that find them believe happened in the past (although I'm not sure about this part and might be conflating it with another book)
-- These 'sections' are either different parts of a long book or several shorter books in an omnibus edition. I remember it being a thick book.
-- I read it in my school library so it would either have been teenage fiction or adult fiction that a school librarian wouldn't consider totally unsuitable for teenagers!
The part I remember specifically is that a small group of young people encounter a society that's either all female or very matriarchal. They're also very eco-conscious and live in harmony with nature (possibly they have houses in the trees?). The girls or young women in the group are invited to stay with this society but they ultimately decide not to because the male(s) in the group either wouldn't be allowed to live there or wouldn't be treated equally.
It isn't The Gate to Women's Country, which is what I get when I search, along with newer books. The women's society was only one part of the story
Thank you for any suggestions!