Polkadot Paper
This is the initial technical vision paper for the Polkadot network protocol, the heterogeneous extensible multi-chain.
Polkadot allows new designs of blockchains to communicate and pool their security while still allowing them to have entirely arbitrary state-transition functions. This opens to the door to an "internet" of blockchains, where private and consortium chains can be firewalled from open and public chains like Ethereum without losing the ability to communicate with them on their own terms, not unlike the intranet/internet synergy we see today.
Find out more and read the paper.