"Communicating Sequential Processes" by Tony Hoare: https://www.cs.cmu.edu/~crary/819-f09/Hoare78.pdf

It had intrigued me due to its promise of designing lock-free concurrent systems, that can (I think) also be proven to be deadlock-free.

You do this by building a simple concurrent block that is proven to work correctly, and then build bigger ones using the smaller, proven blocks, to create more complex systems.

The way it is designed is processes don't share data and don't have locks. They use synchronized IPC for passing and modifying data. It seemed to be a foundational piece for designing reliable systems that incorporate concurrency in them.