Reddit reviews The Encyclopedia of American Soccer History

I haven't read it, but I know the author of this is fantastic (he wrote most of this website.)

https://www.amazon.com/Encyclopedia-American-Soccer-History/dp/0810839806

You can check here for their list of what books they think are best.

http://homepages.sover.net/~spectrum/books.html

Some titles that come to mind:

• Soccer in a Football World: The Story of America's Forgotten Game
  by David Wangerin
  
  
• Distant Corners: American Soccer's History of Missed Opportunities and Lost Causes
  by David Wangerin
  
  
• The Encyclopedia of American Soccer History
  by Roger Allaway, Colin Jose and David Litterer
  
  
• Corner Offices & Corner Kicks
  by Roger Allaway
  
  
• North American Soccer League Encyclopedia
  by Colin Jose
  
  
• Offside: Soccer and American Exceptionalism
  
  by Andrei S. Markovits and Steven L. Hellerman
  
  
• Long-Range Goals: The Success Story of Major League Soccer
  by Beau Dure
  
  
• Star-Spangled Soccer: The Selling, Marketing and Management of Soccer in the USA
  by Gary Hopkins
  
  
• Once in a Lifetime; the Incredible Story of the New York Cosmos
  by Gavin Newsham
  
  
• Love Thy Soccer (pre-order)
  by Sean Reid