I didn't know her, but sometimes I'd walk by her brownstone on 20th Street (a few blocks from my own building on 21st) and see her through the window, drawing. I always loved that.
Holland Cotter, writing for the New York Times, reports it was a heart attack. Read his obit in the New York Times and my own blog post from 2008 on her Guggenheim retrospective. (Next week I'll have images from Mind and Matter, the exhibition currently at MoMA, which features the artist's carved wood sculptures and a fabulous cloth book. )