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.