Diamond Contract which is called EIP-2535 is the most advanced upgradable smart contracts. It is multi facet proxy contracts which allow developers to upgrade easily already deploy contracts.
It has lots of valuable articles and turtorials. This repository provides you lots of resources to study Diamond Contract.
1. EIP-2535 Diamonds
2. EIP-2535 Diamonds Reference Section
3. Understanding Diamonds on Ethereum
4. Solidity delegatecall documentation
5. Layout of State Variables in Storage
6. How Diamond Storage Works
7. Solidity Libraries Can't Have State Variables -- Oh Yes They Can!
8. AppStorage Pattern for State Variables in Solidity
9. How to Share Functions Between Facets of a Diamond
Feel free to make pull request to add more valuable resources.