TaPL really falls down when trying to bootstrap your way to understanding the notation. A lot of the notation and theory revolves around, essentially, implementing a concurrent virtual machine. I like the original algorithm W paper because it doesn't gloss this conceptual step: it is very much a virtual machine & you can see the authors handling the edge cases. The operational semantics in TaPL are (frankly) obtuse. Also, TaPL makes it seem like new features can be desugared to old features — and they can — but a little more prose explaining the feature's behavior directly without just tossing you into the semantic deep end would've made a much nicer text.