Trumpeter Miles Davis - his life and music, Miles Davis: A Biography, by Ian Carr. New York: William Morrow & Co. 324 pp. $14.95.

''You have to think when you play; you have to help each other - you can't just play for yourself.'' This from one of the most individualistic if not iconoclastic of jazz musicians, trumpeter Miles Davis.

It is a sample of the way this book tempers the image of the lonely, sometimes reclusive genius who forces audiences to meet him on his terms and builds up a remarkable body of work over the erratic decades. Yet the exhortation to thought and unselfish interplay, a minidefinition of jazz at its best, is fully in tune with the landmark Davis recordings in various styles. Mr. Carr, a musician himself, turns much of the book into a searching record review, as he evokes the sometimes spontaneous, sometimes lengthily prepared recording sessions and their results.

The biographical details, evidently gleaned primarily from the public record, are related to the ups and downs of the music. Entangled in drugs for a time like other members of his band, Davis came to see the folly of it and quit cold turkey. He could lash out profanely at racism and other targets. Personal and musical moods could coincide. The recent Davis ''comeback'' to playing in public has been disappointing in some ways. But here are reminders of the aspiration and professional skill that we surely have not seen the last of.

''Miles, rather than play the composition, he wants to play the conception that the composition came from,'' says pianist Herbie Hancock.

''That's what I tell all my musicians; I tell them to be ready to play what you know and play above what you know,'' says Davis. ''Anything might happen above what you've been used to playing. . . .''

To an accomplished guitarist he says, ''Play it like you don't know how to play the guitar.''

To a pianist he says, ''Don't finish your idea; let them finish it for you.'' And:

''End your solo before you're done.''