I really feel like putting up Dosbox, buying a copy of "Advanced MS/DOS", using the Turbo TASM tool and working through that is better training. I think it took me basically a month of evenings to cover that book.

The thing is - when you do that, you'll pretty much be able to form a model for any API you ever see. As a buddy used to say, "It's all ioctl()".

Then go the larger-word instruction sets.