For the record, my order started to finalize today. I received a notice that part of my order was shipping out. Indeed, it has been split.
It's also worth pointing out that when I added up the Subtotal, Sales Tax, and Shipping, the total was actually $5.65 more than the total that was listed. As far as I can tell, this $5.65 was the cost to ship that one book separately.
I created a new thread down below, but I'm posting here because I saw a bunch of people that had their order split; those of you that did might want to go check to see if it added an extra shipping cost. You might have to do the math yourself, despite what the listed order total says on the page.