One good book I remember using back in the late 70's/early 80's was "Medicine for Mountaineering". It was good because it went far beyond basic first aid. I seem to remember it including indication/contraindication/dosage information for medications that a non-medical person would not normally have access to. But if the zombies have come, and you have no doctor around (but you do have a pharmacy to pilfer), then such information would be useful. I don't know if this book is still published, or if the lawyers have gotten a hold of it and stripped out this advanced information, but if the book still exists as I remember it then it might be a good one to pick up. Caveat: When I was reading this book, I was a paramedic, so not totally what you'd consider "non-medical". The book might be a tad much for the truly non-medical.

Alternately, you could buy something like "The Nurses Drug Handbook" for when you need to play medical provider if you're truly away from civilization with no other options (still assuming you have a pharmacy you can pilfer, and at least somewhat of a brain to apply to the situation). The standard book on drugs may be "The Physician's Desk Reference" (called the "PDR" for short), but that thing is huge and expensive (unless you buy a version that's a few years old).