What is functional programming and why should I learn it?
The most pragmatic explanation about what FP is and why it’s useful I have read is What is functional programming by Kris Jenkins. Robert C. Martin gave a very good introduction to the topic at NDC 2014 conference.
What is Category Theory?
I can recommend Category Theory for Programmers by Bartosz Milewski. He has not yet finished this series, but you can read the work-in-progress on his website already.