Lamport's 'Time, Clocks and the Ordering of Events in a Distributed System'. Very readable and, if you are new to distributed systems, really fast way to get a useful new mental model.
https://www.microsoft.com/en-us/research/publication/time-cl...
https://www.microsoft.com/en-us/research/publication/time-cl...
"A distributed system is one in which the failure of a computer you didn't even know existed can render your own computer unusable." Leslie Lamport