The key part for me here is the characterisation “internet writing”. I would argue that the Substack Book is not always literally Substack posts sewn together (though that can be an element of it), but books written with the affect of Substacking: piecemeal, personal, presumptive of the audience’s pre-existing interest in the author.
It occurred to me while reading it that what I was holding belonged to a new genre: the Substack Book. Like the blogs of the aughts, Substack has already become its own thing: it incentivizes a certain mood, feel, and style; it has brought into being a recognizable literary form. You know it when you see it.
Substack co-founder Hamish McKenzie will this year publish his own book, How to Save the Media . It will likely re-examine topics he has already covered on his newsletter Disjointed. It may well even include reworkings of that content. It will almost certainly be, according to my definition above, a Substack Book.