I first learned about the Delphi Method from
the book Software Estimation: Demystifying the Black Art.
Although I have not used it in practice yet, I like the concept, especially for some more difficult (bigger) estimation tasks, so I am mentioning it in my book besides other "group estimation techniques".
It is true that this is basically async "planning poker".
I first learned about it when a client hired me to write software to support this method for use in pharmaceutical planning and marketing. Very useful if done correctly.
Although I have not used it in practice yet, I like the concept, especially for some more difficult (bigger) estimation tasks, so I am mentioning it in my book besides other "group estimation techniques".
It is true that this is basically async "planning poker".