Complex sentence: She continued performing mostly in theatre until the mid 1970s, when she began a second career that continues to this day as a cabaret and concert singer and has made numerous recordings, some of them winning Grammy Awards.
Split into simpler sentences: She continued performing mostly in theatre until the mid 1970s, when she began a second career that continues to this day as a cabaret and concert singer. Over her career, Cook has made numerous recordings, some of them winning Grammy Awards.