#1 (https://www.dropbox.com/s/bczmu8kjfhn6ymr/bug1.mov): Scrolling is actually faster in the upward direction than the downward direction. In this video, I continuously move my fingers on my trackpad up 1cm and down 2cm. You would expect the same movement to scroll equivalent distances, but it does not. In fact, you will see that sometimes it stutters or will not scroll at all. (There is nothing wrong with my trackpad, and the problem only affects your site.) This problem affects both Chrome and Firefox.
#2 (https://www.dropbox.com/s/x9zzd298v85atf9/bug2.mov): The viewport is often mysteriously constrained and refuses to scroll any further, even though there is more text. In this video, I opened a fresh window, immediately clicked on one of the chapters, and tried to scroll downards. You'll see me struggle for a bit, then scroll to the previous chapter and then back to the original chapter. Only after doing this am I able to scroll downwards. Seems to affect Chrome only.
Looks promising aside from these rather severe issues.