Yumi Tanimura was very popular among young males especially in the early 90s. She was famous not only as a singer-songwriter but also as a radio host.
Most of her albums climbed the charts, but few of her singles didn't become big hits, so she isn't so well-known among the public now(this is my personal impression).
She was a big fan of fusion-jazz (she is also a good piano player), and so she performed with Casiopea several times.
She is now married to the president of McDonald's Japan, and have a few concerts a year.
Some former fans still criticize her because she was lying about her age, but that is not the point. I think it is definitely true that she was great as a songwriter.
Most of her albums sold so well, so they are available at very cheap prices in Japan.
http://ja.wikipedia.org/wiki/%E8%B0%B7%E6%9D%91%E6%9C%89%E7%BE%8E