Where is standard American English derived from?

I have a book that explains how to speak in standard American English (American Accent Training — Barron's).

What does the term standard American English refer to?

Is there a region in the United States of America that has a pronunciation similar, or closer to standard American English (in the same way standard Italian is derived from a dialect spoken in Tuscany)?

Answer

“Standard” American English (SAE), when used to describe accents, is identical to the term “General American English”, and means American English spoken without any clear regional dialect markers. Sociolinguists tell us that this means essentially English spoken in the north midlands region, like Iowa, but in reality it is spoken all over the country, even in places where many people speak with what others would describe as “heavy” or “thick” accents. SAE is the English used by most television and radio broadcasters—that is, those who don’t speak with an identifiable regional accent.

In truth, a “standard” American accent is defined more by what it isn’t than what it is. It does not have the Northern Cities Vowel Shift, New England non-rhoticity, features of New York English, or any of the phonological features of Southern American English.

There is some debate as to whether speakers with or without the cot-caught merger (that is, whether or not the words cot and caught are pronounced differently) are speaking SAE, but most would consider that not a defining characteristic of SAE.