theory11 — Magic Tricks & the World's Finest Playing Cards

Who is Erdnase?

He might have been more than one person.

Could've been. Or atleast, the book was written with two or more people. Certain context clues can lead us to this conclusion, like the following quote from the SWE Shift:

S.W. Erdnase said:
We have not dubbed the following process with our initials
because we wish to appear "big on the bills," but merely to give it a
name.
 
It's still a likely conclusion which is just as plausible as a single author.

Yeah, definitely. I just wanted to add that as a bit of extra info. Personally, I don't buy the whole two author thing for a variety of reasons, but it's possible Erdnase had advice and some contributions from a second person.