Well, you can't have a book on poisons without also having a book on antidotes, or, if not antidotes, at least medicines or remedies or whatever you want to call them. I mean, though we might not actually ever write about a character getting a paper cut or the flue, they probably do/have during the course of their fictional lives, and we, the writers, should probably know the basic remedies for those ailments when and if the time comes to write about them.
So, National Geographic Guide to Medicinal Herbs: The World's Most Effective Healing Plants is just one of many different books you could put on your bookshelf about health and healing. You could always go with a more medical journal, I think one or two of those makes my list at some point too, but for those of you writing about the middle ages or even earlier in time, then you might want to know what herbal remedies would be available for your characters. (But be aware that those names of herbs from the middle ages might have changed in modern times, and not all herbs grow everywhere. Good thing my bookshelf is large seeing all the things I'm going to be filling it up