Bitcoin Developer Guide (Bitcoin.org)
Bitcoin Glossary of Terms – Bitcoin Magazine
Stats and Visual Tools
Bitnodes: https://bitnodes.io/
Mempool: https://mempool.space/
Bitcoin Explorer: https://bitcoinexplorer.org/
Lightning Network Explorer: https://explorer.acinq.co/
1ML – Lightning Network Search and Analysis Engine: https://1ml.com/
Where can you spend Bitcoin? BTCÂ Map: https://btcmap.org/