http://www.isfdb.org/cgi-bin/ea.cgi?105). That will give you an idea of just how staggering her oeuvre is. I should probably add that she won the World Fantasy Award, British Fantasy Award, and the Nebula Award multiple times.
More important to me, though, than that adjective "prolific" is the fact that her continual production was not repetitive, but a mining of particular thematic veins, which is, of course, what artists do. Her Birthgrave trilogy was among the first books of fantasy I ever read, and, along with C.J. Cherryh's Morgaine series consequently shaped my idea of what fantasy ought to be (which meant that most other fantasy I encountered simply did not measure up). And of course her few science fiction novels formed part of my feminist sf reading foundation along with Russ, Butler, Charnas etc.
You might be able to imagine, then, how I felt when a manuscript arrived in the mail from Tanith, here at Aqueduct a few years ago. I was surprised to learn that she still worked with a typewriter--because electronic screens triggered her migraines. For that reason, her husband mediated email communications for her, and we had to do the editorial process the old-fashioned way, via snail mail to England. Kath and I were happy to do so, for she was a joy to work with and we loved her stories.
I will be rereading some of her work this summer, I think--as well as reading work that will be new to me. Her status as "prolific" is a great blessing.
I'll conclude with the words now up on her website:
Though we come and go, and pass into the shadows, where we leave
behind us stories told – on paper, on the wings of butterflies, on the
wind, on the hearts of others – there we are remembered, there we work
magic and great change – passing on the fire like a torch – forever
and forever. Till the sky falls, and all things are flawless and need
no words at all. --Tanith Lee