Another student written handout is here. It is essentially a book completing everything about Functional Equations, from basic to IMO level. Where it differs from other resources is that it shows the motivations and thinking of the solutions and not just the answers, which really helps in problem solving. The author of this handout is Pang-Cheng, Wu who himself is a Silver medallist in IMO. Have fun solving!!
The above work is written by Pang-Cheng, Wu, so all credits to them. Cheers!