For Deeper Understanding
Original Paper (very technical):
"The Byzantine Generals Problem" – Lamport, Shostak, Pease (1982)
Link: https://lamport.azurewebsites.net/pubs/byz.pdf (Read just the abstract and introduction)
Accessible Explanation:
"The Byzantine Generals Problem – Simply Explained" – Various YouTubers (search)
Bitcoin Whitepaper Reference:
Satoshi Nakamoto's "Bitcoin: A Peer-to-Peer Electronic Cash System" – Section 2
Link: https://bitcoin.org/bitcoin.pdf