I like your description of Fitts Law and the descriptions of what each variable means. One improvement for the chapter might be to remind the user about the a and b coefficients in the paragraph after the apple menu bar picture. I found myself having to go back up to the paragraph where you first described the coefficients to remind myself what they were.
I believe a graphic of a person trying to reach a target might have done better for the Fitts law description. While I understood the description in the reading with the current image, it took me a second to match the example of "how far someone has to move their finger" with the mouse cursor shown in the image. Hence, I believe either changing the example described to match the image or vice versa would reduce confusion.