Working on a little essay to post here. In the meantime, here's a little quote from Kahnweiler's, Der Weg zum Kubismus [translated as The Rise of Cubism] (1920) to whet your appetites:
"At this point, Braque's introduction of undistorted real objects into the painting takes on its full significance. When 'real details' are thus introduced the result is a stimulus which carries with it memory images. Combing 'real' stimulus and the scheme of forms, these images construct the finished object in the mind. Thus the desired physical representation comes into being in the spectator's mind....
"This new language has given painting an unprecedented freedom. It is no longer bound to the more or less verisimilar optic image which describes the object from a single viewpoint....
"Our a priori knowledge of [geometric] forms is the necessary condition, without which there would be no seeing, no world of objects.... Humanity is possessed not only by the longing for these lines and forms, but also by the ability to create them."