https://en.wikipedia.org/wiki/Pharmacopoeia
https://en.wikipedia.org/wiki/WHO_Model_List_of_Essential_Medicines
What I've had in mind (for at least five years...) is a book that would contain a list of essential medicines, as well as the plants that contain precursor chemicals, how to extract them, and the synthesis steps to produce the medicine. So, it would tell you what plants contain salicylic acid, how to isolate it, and the steps needed to convert it to Aspirin. Another example would be how to produce Ether from alcohol. It was removed from the essential medicines list in 2005, but a post-apocalyptic group probably won't be able to avail themselves of its replacements...