A master course by Benoit Fresse on [operads and Grothendieck-Teichmüller groups][1] (in french), at Université Lille 1, given this semester (Winter 2012).
The course has a really nice and complete introduction to the subject. The principal reference is a [preprint][2] (in english) writed by Fresse.

  [1]: http://math.univ-lille1.fr/~operads/LilleCourse2012-Synopsis.html
  [2]: http://math.univ-lille1.fr/~operads/pub/OperadGT-January2012Preprint.pdf