Not unusual, but you used them with taste and restraint, like the rest of your layout and animations. That's something that HN comments like, I think. Notice the distinct lack of "OMG some fancy presentation trick ! Litteraly unviewable!" comments that often happens when an unusual layout is presented (and often with reason; but sometime to a fault).

I guess the main praise your page UI is that it looks, well, like a page. But augmented I guess ?

Personnally I really like the way you used a grid to separate the content from the nav. I like that you used both the left sidebar for nav and for the header number (re-using the same space for multiple purpose feels elegant, because those purpose are secondary to the content, if that makes sense). And I like that the grid anchors your eyes by fencing the different chapters along with the nav. (and now that I mention it, it feels weird that the headings are outside their chapters, but it didn't felt like that upon first reading).