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Field guides and preparations are generally separate. Peterson's makes an excellent medicinal plants and herbs guide... Amazon
Dosages and preparations require a high degree of cross-referencing, as there is a lot less scientific research in this area (at least that is how I go about it)... Start with foraging around for what is available to you (using a field guide). Then hit up Wikipedia and work your way through the references, look at books on integrative medicine and natural health. Sites like Wikipedia and Web MD, will tell you if there is immediate or significant long term danger of consuming the herb, and give some insight into the alkaloids and active ingredients of most common medicinal herbs... After I pull from places like J-stor, and dive into more esoteric sources like Victorian pharmacopeias and folk lore, and books on ethnobotany etc.
Edit: conservation areas are also a good place to get started, MNR (or whatever the USA version is), and park departments often print little guides to local plants and their history.
Pearson has a good field guide you can check out. It's divided into east and west volumes for North America. It's a generic title like "Medicinal Plants of North America"
Edit: amazon link https://www.amazon.com/dp/0547943989/ref=cm_sw_r_cp_awdb_e2zwzbZEB5TXC
Peterson's Field Guide to Western Medicinal Plants and Herbs
https://www.amazon.com/Peterson-Medicinal-Eastern-Central-America/dp/0547943989