I learned some basic kappo techniques while studying DZR and judo years ago. In general, they work surprisingly well, but it is ALWAYS better to rely on modern First Aid/CPR for real life emergencies.
Exceptions where I found kappo very useful and effective:
1. stopping mild head bleeding, particularly from bloodied and/or broken noses and split eyebrows - generally effective in 2 minutes or less, with little/no pressure applied to the wound - I've used this more times than I care to remember, both on myself and on many others.
2. resetting a broken nose (my own), in which the cartilaginous tip had merely been dislocated, as opposed to one involving significant damage to the nasal bones - had it checked at the ER later that night, and they said it was as good as they could have done
3. reviving someone woozy (but conscious) after being choked out - sometimes people are slow to recover their wits, and kappo includes a few methods for quickly getting them back up to speed
DO NOT attempt to learn kappo from a book. Get an experienced instructor for hands-on training. It is possible to F someone up if you get it wrong, like any other medical care.
That said, Darrell Craig's book "Japan's Ultimate Martial Art" includes some pages on kappo, including a description of the nose-setting technique I used.