What is your good list of more than 20 math books for functional equation with 200+ pages of each? And not very specific book with very complicated/very sophiscated topic for mathmatician please!
It is very hard to find those books at a level between advanced undergraduate to beginning graduate level these day. So i am here invite you to share some love from being share your technical experience through your past learning experience. I and the whole world wide undergraduate to graduate students would greatly appreciate your effort.