What were the "Prefaces to Shakespeare"?
Harley Granville-Barker was a critic, director, actor, producer and playwright (his most famous plays being, perhaps, "Waste" and "The Madras House"). He wrote a series of "Prefaces to Shakespeare", introductions to specific Shakespeare plays, which tended to focus on the dramatic possibilities which Barker had gleaned from his work in the theatre.
They were originally published between 1927 and 1947, after Granville Barker's own productions had marked a revolution in his Shakespearean productions during the first part of the twentieth century by focussing on the way that Shakespeare was directed produced on the Elizabethan stage.
G. B. Harrison said that Granville Barker's were "the most important productions for a hundred years not only because they were beautiful in themselves, but because for the first time since the seventeenth century Shakespeare's plays were played just as they were written, and not cut and rearranged to suit the scene-shifter".
Here's T.S. Eliot talking about the prefaces:
Perhaps more than any other single writer, H. Granville-Barker by his prefaces, illuminating the plays with the understanding of the producer, has suggested the need for a synthesis of the several points of view from which Shakespeare can be studied.