Genius Within: The Inner Life of Glenn Gould is a fantastic documentary now running on PBS (and available for free viewing on the American Masters web site). Gould was perhaps one of the most important performers of the 20th century. In addition to being one of the most brilliant and original pianists of all time, he also understood media better than anyone else in his time, and on top of that, is one of the most eccentric personalities in music.
Even if you have never heard his music, he is a fascinating character. A recluse and a hypochondriac who was borderline personality disorder, he was one of the most popular classical soloists in the 50s, yet, in mid-career, he decided that he would be better off never performing publicly again. Instead, he decided to communicate with his audience only through recordings. The movie only scratches the surface of his bizarre personality and unique beliefs about art and audiences. Of course I also highly recommend 32 Short Films About Glenn Gould, which present even more strange elements of his personality.
On the other hand, one of the reasons that Genius Within is so interesting is that it breaks many of the myths about Gould. It was forever thought that he might be gay and in the closet, or perhaps even in denial, or perhaps might have been completely asexual, but a biography in 2007 revealed that he had had a long term love affair with the wife of a famous composer/performer, and out of respect, no one ever discussed it publicly. She is interviewed in this film. It is also revealed that many of his eccentricities were exaggerated to create publicity for him. Nonetheless, they all do seem to have originated in fact, and did get worse as he aged, dying young at age 50.
If you have even a passing interesting in the subject, watch this, it's great.