The American Language
An Inquiry into the Development of English in the United States
– H.L. Mencken
This classic was written to clarify the discrepancies between British and American English and to define the distinguishing characteristics of American English. Mencken’s groundbreaking study was undoubtedly the most scientific linguistic work on the American language to date and continues to serve as a definitive resource in the field.
I had no idea this was online. Highly recommended.