I love plants and gardening, and being a practical sort, I’d like to know more about the plants I grow. I’m not looking for sorcery spells or “cleans out the humors” types of resources, but I’m very interested in literature that has ways to use plants (teas, infusions, etc.) that are scientifically backed.
Aspirin / willow bark is the classic example. I’m looking for a big, fat book of that kind of stuff.
Herbal Medicine: Expanded Commission E Monographs
Science-Based Medicine: Guide to Herbs, Supplements and Nutrition
Herbal Medications: An Evidence-Based Review
Herbal Medicine: Biomolecular and Clinical Aspects, Second Edition (Once you’ve recovered from the sticker shock at that link, realize that this one is available online for nothin’. 
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If you’re interested in a more practical medicine making book that has some remedies, but is mostly about how to make the medicine from your plants, The Herbal Medicine-Maker’s Handbook is the best one I know.
Thanks, WhyNot. 
Since you didn’t specify fiction or nonfiction . . . read Outlander yet?
Susan Witing Albert’s China Bayles herb shop mysteries are excellent.
Just to give you a bit of a different take on the answer I’d recommend The Drunken Botanist. Its a fun read about plants that you can use to make booze.