JONI MITCHELL: Ladies of the Canyon. Joni Mitchell, vocals, guitar, keyboards, and arr. (Willy; Blue Boy; Conversation; nine more.) Reprise 6376, $4.98. Tape: 4RA 6376, $5.98.

It is a peculiar fact of today's music that those who perform best also tend to write the best songs. Witness the Beatles, Simon and Garfunkel, Crosby/Stills/Nash/Young, Laura Nyro. Today's product is intensely personal, wedded to itself. Of the many remarkable self-contained talents, Miss Joni Mitchell soars highest. While all of her gifts are superb, I suspect that no one part would work without the others.

There is no producer listed on the album because Miss Mitchell has produced it herself. I once wandered into one of her recording sessions by mistake. No one was there but Miss Mitchell and her sympathetic engineer-adviser, Henry Lewy. She even painted the album cover, which includes a simple, accurate, line-drawn self-portrait.

The music is also all Joni Mitchell - melodies, lyrics, voice, guitar, keyboard, arrangements. A few tracks include a touch of the outside world - a clarinet here (and I wish he'd listen to the lyrics instead of jazzing up his moment), a cello there, and one tune that uses a few friends as a background chorus.

This is Miss Mitchell's third album, and the first on which she plays piano. By some mysterious process, she makes it sound like a guitar. Her playing is equally dramatic, strange, and uniquely harmonic on both instruments.

In the interest of balance, a few inconsequential imperfections may be noted. Miss Mitchell's otherwise crystal voice wobbles a bit on high notes, and a couple of songs - Morning Morgantown and Circle Game - must have been written some time ago, before full flower. But then, early Mitchell suffers only when compared to prime Mitchell.

This young lady has never been more in her prime than when she wrote the incredible Woodstock, in which she plays a Fender-Rhodes electric piano. The song captures all the dreamlike beauty one likes to think must have existed in that quite real weekend in the country last summer. Another song, unmistakably autobiographical, tells of Miss Mitchell as she comes upon a street-corner musician playing his clarinet, ignored by passersby. "Now me I play for fortune and those velvet curtain calls....And I play if you have the money or if you're a friend to me. But the one-man band by the quick lunch stand, he was playing real good for free." Other gems: The Arrangement, Big Yellow Taxi, The Priest.

Miss Mitchell is available, unfettered, on Reprise. I can't recommend anyone more highly.