Her masterpiece, I think, is pretty clearly The Female Man. (It's been a long time since I read it. I should read it again. I think I missed a lot of it first time around.)
"The Female Man" is one of those titles (like The Crying of Lot 49 or Stuck Rubber Baby) which make no sense when you first hear it but, once you understand it, is impossible to unhear: it seems so bloody obvious what it means. (As opposed, say, to those titles that take on new meanings after you read the book, but are perfectly comprehensible before.) Unlike those other two examples, however, its meaning is not a spoiler, so I thought I'd share it. From The Female Man:
"Man" is a rhetorical convenience for "human." "Man" includes "woman." Thus:...and then you get it: "Female Man". Right. Of course. -- And, if you're like me, you feel ashamed you didn't get it earlier.
1. The Eternal Feminine leads us ever upward and on. (Guess who "us" is)
2. The last man on earth will spend the last hour before the holocaust searching for his wife and child. (Review of The Second Sex by the first sex)
3. We all have the impulse, at times, to get rid of our wives. (Irving Howe, introduction to Hanly, talking about my wife)
4. Great scientists choose their problems as they choose their wives. (A. H. Maslow, who should know better)
5. Man is a hunter who wishes to compete for the best kill and the best female. (everybody)
(Not that this captures the whole meaning -- the term has more meanings than that -- but this gets at its essence, I think.)
It's probably wrong of me, but I take a small piece of consolation in the fact that the parenthesis for number 4 -- where Joanna Russ talks about "my wife" -- now sounds far more natural than it did when the book came out 36 years ago; and that I suspect (or at least hope) that twenty-year-olds who read it 36 years from now won't get why it was supposed to sound odd at all.
She's a great writer. As always in such cases, the small sliver of consolation is that her works remain. But it's always hard not to wish that won't be more of them.
Update: Links to some of Russ's work online:
• Russ's Nebula-award winning short story, "When it Changed", set in (one of the) worlds that shows up in The Female Man (or, at least, in a version of it), published three years before the novel (so not a spoiler). (via)
• Russ's 1975 essay "Towards an Aesthetics of Science Fiction" (via Mumpsimus, as is the following one)
• The 2006 phone interview with Samuel R. Delany at Wiscon (referenced by Patrick Nielsen Hayden in the above-linked thread).
• The opening of her book How to Suppress Women's Writing.