Peter Guralnick’s Sweet Soul Music: Rhythm and Blues and the Southern Dream of Freedom, published in 1986, is a perfect sum on southern soul music, focusing on Stax Records’ story. Nearly 30 years later, its value is unquestionable.
Though one can blame the book for dismissing northern Detroit soul music or Philly Sound, as did Joel Rudinow (Rudinow, 2010) :
In the end, Guralnick draws a bright geographical line, emphasizing audience demographics as well as regional-cultural and musical-stylistic differences, and confines himself to the project of reconstructing the oral history of southern soul music. For Guralnick, Motown is not soul music. His argument for this is partly business-historic and partly stylistic.1
Nevertheless, it appears very well built and documented. What could have been a boring story of hit makers appears to be an accurate depiction of how this unique “Memphis sound” business was shaped : while still calling some records “race records” (this expression is very typical from the 30/40s era), the Stax enterprise was mixed, until its most noticable writing team : Otis Redding and Steve Cropper.
Sweet Soul Music has turned into a classic introduction to soul music, despite its paradigm, chosing not to consider northern Motown music neither as rhythm nor as soul music, but… as pop music. Its authors index is a gold mine, and the sources are reliable.
This book brings the material and opens a path for building a history of soul music under a “black and white” approach, overflowing anecdotes that illustrates the subject (never too much).

1 Rudinow, J. (2010). Soul Music : Tracking the Spiritual Roots of Pop from Plato to Motown. University of Michigan Press. https://muse.jhu.edu/book/1103