This response (and the whole thread) may be useful: ReScript and the future of ReasonML - ReasonML Forums
As far as I can tell, as of now ReasonML and ReScript are aiming for different use cases.
Since both ReasonML and ReScript are niche languages, I don’t think there would be a lot of support for an update to the book. If there were, I would write it for ReScript, which is focused on the web.