You could start with any book written by Bruce Catton or Shelby Foote for an overview of the Civil War.
Local library may have some or all of their works.
Both authors have extensive bibliographies at the end of their books so you can get ideas of what to seek next depending on what interests you--battles, leaders, regimental histories, issues etc.
I'd stay away from any book written by Stackpole. They have numerous errors of fact. Books published by
Stackpole are ok.
Fletcher Pratt's "A Short History of the Civil War" is my favorite one-volume history of the war. I have had a paperback copy for more than 50 years so I don't know how common it is now.