One fascinating aspect of Boulez as a composer is that he often reworked his works. He had many pieces that he considered to be unfinished.
One such piece is Notations, a piece he first wrote in 1945. Then it was 12 small piano pieces. Later, in 1980, he reworked and expanded the pieces to orchestra pieces. Compare the piano version of Hiératique and the version for orchestra.
Another example is Anthèmes where the first version was a solo violin piece written in 1991. Later in 1997 he expanded and turned it into a piece for solo violin and live electronics, realized at IRCAM.
Below is a interview that shows a very kind, intelligent and warm Pierre Boulez talking about his music.