7 The Idea of Standard American English by William A. Kretzschmar, Jr. and Charles F. Meyer

I was thinking to improve English pronunciation but mistakenly checked out this book... Anyway, Chapter 7 (pp.139-158) is very interesting.

7.1 Introduction
Codification is not the writing down of what people actually say and write, but the formulation of rules for what people should say, often based on cultural factors like the political influence of the capital.（139）
The first full 'American' treatment of American English did not occur until 1919, when H. L. Mencken published the first edition of The American Language.（141）
Standard American English...is not identical with the language of any natural population of speakers; it is a very real institutional construct that has attracted the loyalty of a committed group of speakers who claim that they speak it.（143）
it is not in fact all one thing, but instead is characterised by a mixture of features that come from people's experience with Standard American English in their particular regional and social circumstances.（143）

7.4 Characteristics of Standard American English
For pronunciation, SAE is best defined as the avoidance of pronunciations associated with particular regions or social groups. （151）
the most highly educated speakers in formal settings tend to suppress any linguistic features that they recognise as regionally or socially identifiable ('marked' features).（152）

7.5 Perceptions of SAE
the demographic change that has occurred in America after World War II. The urban demographic pattern changed from residential neighbourhoods within cities to the model of an urban core surrounded by suburbs....（153）Weak ties in social networks tend to promote the transmission of features from group to group, not the maintenance of strongly marked features within a population group. At the same time, the most highly educated segment of the population became more mobile nationally, owing to improved transportation (highways, airlines.).
The only problem with the dichotomy between SAE and 'non-standard' English is the assumption that SAE has an empirical basis in language in use. It does not.（156）