Archive.org has a couple older manuals digitized. The first was published in 1940 by Edwin Haislet, and later became the Navy's standard boxing manual. Haislet directed the Northwest Golden Gloves during this period, and ran the University of Minnesota's boxing program. The second was published a decade later by Jack Dempsey. You're probably all familiar with Championship Fighting, but it's a useful little book. Note that both books contain some differences from the standard amateur techniques accepted today. That alone makes them fairly interesting. Haislet: http://archive.org/details/Boxing Dempsey: http://archive.org/details/ChampionshipFighting
The Dempsey book is interesting because it's largely concerned with the teaching of boxing for self-defense.
