Maybe I'll write a book about Improbable some day - but more likely about the mythical origin story :) That said, at two different times I managed teams that made demos, tutorials and documentation, most recently https://twitter.com/gabrielgambetta/status/92135876311796121..., but there's people much more qualified than I am to write technical stuff about SpatialOS nowadays.
Region boundaries are a fascinating topic, see this for example: https://improbable.io/games/blog/distributed-physics-without...
But as Woody Harrelson would say, "let's keep it about Computer Graphics, people..." ;)
There are a ton of computer graphics intro books. Not so much on big-world architecture.