Preferably one that doesn’t get into true FP with strong types and such, but focuses more on the rationale behind immutability/referential transperancy.

This is a good paper to read through to get a general understanding of the idea: http://worrydream.com/refs/Hughes-WhyFunctionalProgrammingMatters.pdf I would also highly recommend working with HTDP: https://www.htdp.org/ to get a fuller idea of how the FP concepts link with each other to help you build bigger systems.

Not exactly answering your question, but a good support for learning about immutability and its consequences is IPFS. Once you understand how IPFS works, and what the role of naming (IPNS) is, you just have to imagine how you can do data processing in that universe, and you end up with FP.

The site that got me interest in FP was https://fsharpforfunandprofit.com/