Ballet life behind the scenes; Winter Season: A Dancer's Journal, by Toni Bentley. New York: Random House. 150 pp. $11.95.

''Winter Season: A Dancer's Journal,'' traces the winter of Toni Bentley's discontent - a season spent dancing in the corps of the New York City Ballet and wondering if that's all there is to life.

She gives us some gumdrops of information about life behind the scenes of George Balanchine's company. For example, at one point during the intense Christmas season - 40 ''Nutcrackers'' in five weeks - no one was scheduled to fill in the Spanish variation. She describes Heather Watts running onstage in soft shoes and no makeup, wearing a crooked headpiece and making up her own Spanish variation as she goes along, kicking her partner from time to time.

There are tricks of the trade, such as how to make toe shoes danceable, but the heart of the book is Toni Bentley's chafing and self-doubt in the closed, hierarchical ballet world.

''I once heard a principal dancer say, 'I do my best, and now I'm trying to learn to enjoy it.' '' she writes. ''It struck me that the enjoyment comes last: first one does the work, then one tries to have a good time. I dance best when I have joy already and I dance to celebrate it.''

This book is a chronicle of Miss Bentley's search for the missing joy. The work can be grim. There are exhausting 12-hour days cooped up in the theater. There is always the mirror to confront. There is the problem of being just another corps member. The members of the corps of the New York City Ballet would be soloists anywhere else, she writes, but that doesn't mean much within the ballet. And there's the feeling of being an instrument, played by one of the geniuses of our time, but an instrument nonetheless.

Unfortunately, as she looks for the joy of dancing, she has to examine many bad moods. And the reader feels lost in Miss Bentley's self-absorbed malaise that seems to stretch for pages - and a steamy backstage romance that may offend some.

But at the end of the book, having left the company, Toni Bentley comes back and finds the missing joy. This is a cliche, but its also true. Bentley writes with conviction and freshness, and her triumph is as delightful as it is ordinary, and her book is inspiring for artists or anyone who worries when ''the joy misses out.''