RFC1
Months before the first node of ARPANET went online, the intrepid easy engineers were just starting to discuss the technical underpinnings of what would evolve into the Internet some day. Here, we hear how hosts would communicate to the IMPs, or early routing devices (although maybe more like a Paleolithic version of what's in a standard network interface today). It's nerdy. There's discussion of packets and what bits might do what and later Vint Cerf and Bob Kahn would redo most of this early work as the protocols evolved towards TCP/IP. But reading their technical notes and being able to trace those through thousands of RFCs that show the evolution into the Internet we know today is an amazing look into the history of computing.