When I used to work in DC, I took the Marc train and had plenty of time to read during my commute to and from work. I can recall reading a book my co-worker let me borrow called Deadly Reigns. The book was actually really good, but I don't think it had any design elements that contributed to the easy readability of the book. I'm pretty sure the differentiation between chapters were standard - starting on a new page with a number indicating the chapter. Nothing fancy.
With that said, I don't necessarily believe that the design of the content creates the level of readability. It all depends on the content itself.
