This is a subject of lifetime learning for me and, despite now studying it for many years, I wouldn't dream of tutoring or trying to teach it or even offering application advice - especially not on a forum. Learning the plants and the applications is one thing, learning about extraction processes, the doses, and the diagnostic is a completely different matter. And, this is an ever changing subject; for example comfrey was at one time considered very useful but now advised against.
My emphasis has always been to study the plants in my environment and research what they are useful for rather than study medicinal herbs in general. If I travel, I try and find the common plants or will study the plant species of the area I am visiting.
That said, I am happy to list some of my resources (these are in addition to very good field guides and keys; good identification is a must):
Welsh Herbal Medicine - Hoffmann
Herbs and Health - Peterson
Herbs and Aromatherapy - Metcalfe
The Dictionary of Healing Plants - Dorfler & Roselt
Herbs and Healing Plants of Britain and Europe - (Collins) Podlech
Grow Your Own Drugs - Wong
The Encyclopaedia of Medicinal Plants - Chevallier
Native American Ethnobotany - Moerman
and many more ....
The latter covers North America but I am in the process of cross referencing common species. It's an expensive book but you can access the database upon which it is based for free at
www.naeb.brit.org
Cheers,
Broch