The only work of hers that I know about is what has been published in the U.S. by Viz Publications....
These tittles include:

Promise
Since You've Been Gone
Love Song
Jewels of the Seaside
The Signal Goes Blink Blink
The Skin of Her Heart

Of these, Promise and Since You've Been Gone are available in the Viz graphic novel Four Shojo Stories, and the rest are available in the graphic novel Love Song, also published by Viz.

I own both of these books, and they are very, very good (this is coming from someone who is usually too much of a cheapskate (*ahem*) to buy anything).

If you're a fan of shoujo art and/or Keiko Nishi, I strongly recomend that you purchase them.