I recommend the Physicians Desk Reference and www.galls.com for first responder first aid and trauma kits. Also, one should become certified in CPR/AED and then take the Wilderness First Aid course.
I was a licensed EMT so that’s a skill I’ll use for bartering if the SHTF big time. I’ve got medical supplies as well as a full oxygen tank and mask. Have leg and arm splints, all that stuff.
When you take the EMT course, you have to work at an emergency room and on an ambulance for a period of time. Plus there is a state test to take. I got my license to be a first responder in my subdivision as it was in the country and it took a while for an ambulance to get there.
You can take a lesser course called an ECA (emergency care attendant) and that is less strenuous than the EMT course.