The Bitcoin white paper, "Bitcoin: A Peer-to-Peer Electronic Cash System", is nine pages long. Satoshi Nakamoto posted it on 31 October 2008.
It explains how to prevent double spending without a trusted third party. It uses a chain of proof of work blocks that nodes accept when it has the most work. It also covers incentives, SPV and privacy.
Takeaway: The core design has stayed recognisably the same since 2008.