I read it a few months ago and was really impressed with how easy it was to read.
It starts out with simple stuff, like serialising data as JSON vs XML. But it moves into complex areas - like how replication and WALs work, including different ways of handling consensus when using leader-leader replication and how Spanner needs atomic clocks to handle it.
But even the complex stuff was explained in a way that I understood, which is an immense achievement.
I'm someone who has been doing this stuff for almost two decades without really knowing this is what I'm doing. I used to think what I was going to do was systems level programming like operating systems and maybe the database systems themselves (e.g. postgres, datomic etc.). But for whatever reason my entire career (so far, but I don't see it changing) has been building data systems for businesses and users.
I read the book from cover to cover and half of it was like "ohh... that's how that works, that's what I'm doing wrong" and the other half was "shit, this is something I kinda knew after trying and failing for years, and someone has just written it down in a way I never could".