Malcolm Willoughby's book was "THE" book on how the Coast Guard enforced Prohibition when it was first published in the 1960's, and it remains the most popular work on the subject to date. (It has been re-published several times.) His book inspired me to write my own book on the same topic, although my approach was much different. Willoughby was considered the authority on the subject for his generation, which is why the Coast Guard historian's office keeps stacks of the book in thier inventory. When reading the book, however, keep in mind that it was originally published by the Treasury Department, which the Coast Guard belonged to at the time, so there is a bit of bias and tidyness to many of the accounts. Also, Willoughby was a retired Coast Guard officer. If this book captures your interest, then you will definitely like the newest book on the suject:
Alcohol, Boat Chases, and Shootouts! How the U.S. Coast Guard and Customs Fought Rum Smugglers and Pirates (Part I: 1919-1924)