Scholarship directly focused on the US Consular Service is in short supply. The main book is Charles Stuart Kennedy’s The American Consul: A History of the United States Consular Service, 1776-1914, originally published in 1990 by Greenwood Press. It is now out in a revised second edition that takes the story to 1924; that version was published in 2015 by New Academia Publishing.
Kennedy, a former US Foreign Service officer, provides an account of the institution’s divided temporally and geographically. It focuses on the issue of professionalization and provides many vignettes featuring some of the most famous — or, indeed, infamous — US consuls, such as Nathaniel Hawthorne and James Fenimore Cooper.