Are you looking for technical papers on how they work or practical blog posts on what you can do with them?

For both Bitcoin and Etherium the original white papers do not paint a full picture of the state of each cryptocurrency. Here are a few links I recently read on both BTC and ETH:

Bitcoin Wiki: https://en.bitcoin.it/wiki/Main_Page

Bitcoin BIPs: https://github.com/bitcoin/bips

Etherium Website: https://ethereum.org/ (it has a lot of information spread all over it)

ETH2 Beacon Chain Blog Post: https://ethos.dev/beacon-chain/

There's a lot more literature that you can find via Google but it's spread out all over the place.