What is the best source of documentation for Awk?

the book: "The AWK programming language" written by Aho, Weinberger and Kernighan (AWK)?

You can easily find it online. Prior discussion: https://news.ycombinator.com/item?id=13451454

For GNU AWK you also have https://www.gnu.org/software/gawk/manual/gawk.html

I don't know if it's the best but this resource is good - https://github.com/learnbyexample/learn_gnuawk