Reddit reviews A Druid's Herbal for the Sacred Earth Year

This isn't a book, but I'll share with you the best way I've found to learn herbs. Pick a few that seem interesting. Then get the seeds. Grow some from seed. You get to see how they develop, what the seedlings look like, how it grows, when it flowers, what it smells like. You get familiar with it by taking care of it and spending time with it. Then you use it. Try making a salve. Make tea, or candies. Taste it. Then you'll recognize it anywhere. Next season, do the same with new herbs (but keep the old around as well). Do this even if the herb grows wild in your area. Then you'll be able to spot it easier from a distance just by its overall shape.

Now, on the subject of books: The Complete Book of Herbs is quite nice. But I've found the most useful information in snippets here and there. A Druid's Herbal has good preparation tips, and some good info on the medicinal uses, but goes a lot into magic and other nonsense (still a good read). I have a few pages from various Mother Earth News magazines from the 80's that have great preparation tips. Also, the Internet is great if you know what you're looking for.