p. 260-1, On Paper, Nicholas Basbanes quoting Martin Kemp
" There is no single answer... Different notebooks must be tackled in different ways. Sometimes he'll start with mirror writing at the front but he will quickly turn it upside down if he decides that he's got a different topic, then start at the back. Things appear, reappear some are crossed out, ideas are introduced from other notebooks, and he will interpolate things at later dates. They are extraordinarily erratic and impulsive in how they go about laying down their material, which is both terrifically exciting but also frustrating, because getting a coherent train of thought out of all that is hard work. It's absolutely different from reading most people's notebooks.
There are the intangible things such as the sheer feel of the original that you get, If you're also asking hard questions about the composition of a sheet, of how that sheet developed in the way that it did in the mind, you can often deduce some of that from the density of the lines, the chalk, the different inks that he used; you can get a feel for how the sheets have been worked up in very complicated ways. A facsimile is flat; there is no texture at all to it. People tend to think of paper as a surface on which you simply draw. But it's actually more complicated than that.
When you pick up one of his pocket books, these very fat but very small-in-page-dimension pocket books, you can get a real sense of the urgency and intensity, and the shared mental vigor with which he filled these pages with tiny sketches and tiny bits of writing. You get [a] sense of his thought cascading onward. Then, when you take one of the big double-folded sheets—these were done when he was sitting down in the studio, not outside looking at people and observing his surroundings— you can see a slower and more varied pace. There is ths extraordinary sense of size, and one can grasp the physical nature of it all. But your first impression is the speed at which something has been done."