Oh, geez, it's been a while since I read it. I think it's from the
books by L. Sprague deCamp, about a guy from modern America who
learns to transport himself to magical worlds. The Series is
called something like "Apprentice Adept" (but that's not it,
that's a series by Piers Anthony). Ah! Just found it on
amazon.com. "The Compleat Enchanter" was the original story, but
there were several others as well.
Recently republished as "The Complete Compleat Enchanter", by L.
Sprague deCamp, et. al.
I'll leave it to the curious to read the books and find out who Yvengi
was. :)
Keith Hearn