Reddit reviews The Professional

I've got a number of books I can suggest.

This one's a great history of the sport in the bare-knuckle era, 19th century sporting culture, and the transition to modern boxing.
http://www.amazon.com/The-Manly-Art-Bare-Knuckle-Fighting/dp/0801476089

Probably the best boxing novel ever written. I can't believe this isn't a movie.
http://www.amazon.com/The-Professional-W-c-Heinz/dp/0306810581

The book that got me into the sport when I sat down to read it at a Borders Books when I was 16. An awesome overview of the history of each weight class with over 400 photos and illustrations.
http://www.amazon.com/AN-Illustrated-History-Of-Boxing/dp/0806522011

An excellent anthology of fiction and newspaper articles.
http://www.amazon.com/At-Fights-American-Writers-Boxing/dp/1598532057

The guy spends a little too much time bashing other martial arts, but it's an excellent read about how boxing is useful in a street fight and old school bare knuckle techniques.
http://www.amazon.com/Championship-Streetfighting-Boxing-Martial-Art/dp/0873649346

A superb series of observations from a woman who has spent her life watching boxing. One of the more literary attempts at explaining the primal appeal of a boxing match.
http://www.amazon.com/On-Boxing-Joyce-Carol-Oates/dp/0060874503

I'm not home and don't have access to my library right now, but let me know if you want a few more. I'd say that any one of those is a great gift for a boxing enthusiast. /u/Choccybizzle is right, Four Kings is awesome, and /u/zombiezs suggestion for The Sweet Science is a good one as well. There's a lot of boxing books out there, but you know your friend so you'll know which of those will appeal to him the most.