Resources
Search for "complementary", "mushroom", or "herb"
Consumer Lab
NSF International
US Pharmacopeia
Memorial Sloan Kettering Cancer Center
Natural Medicine Comprehensive Database
American Botanical Council
Cochrane Library
The Mayo Clinic
The World Health Organization
US Department of Health and Human Services, National Institute of Health Sites
Office of Dietary Supplements
MedlinePlus
Dietary Supplement Label Database
National Center for Complementary and Alternative Medicine - Herbs at a Glance
NCBI PubMed
"How Safe is this Product or Practice"
US Food and Drug Administration Sites
Dietary Supplements Information
MedWatch (safety alerts)
Books
Peterson Field Guides indispensable!
The Book of Killer Plants by Kit Carlson PhD
Natural Standard Herb & Supplement Guide by Catherine E. Ulbricht
The Encyclopedia of Medicinal Plants by Andrew Chevallier MNIMH
Family Guide to Natural Medicine by Reader's Digest
The New Healing Herbs by Michael Castleman
The MacMillan Treasury of Herbs by Ann Bonar
Bark by Michael Wojtech
Handbook of Medicinal Herbs by James A. Duke
Any book by Samuel Thayer. He mainly forages for edible plants, but his books are excellent for plant identification and general knowledge.
I have built my library largely from used bookstores and used book websites. Some of these books are out of print and others are expensive so this is the best way to build a library.
Websites
Google Scholar, this is a search engine that looks for scientific books and research papers. There are lots of studies involving medicinal herbs. Hint: if you truly can't make heads or tails out of the terms or charts, or math in the study just read the Abstract at the beginning and skip to the Discussion and Conclusion at the end.
Herbs at a Glance, this is an NIH site of the National Center for Complementary and Integrative Health.
Native American Ethnobotany, A Database of Foods, Drugs, Dyes, and Fibers of Native American Peoples, Derived from Plants.
Cal Photos Many useful photos of herbaceous plants and fungi
Memorial Sloan-Kettering Cancer Center, scientific information about medicinal herbs as integrative medicine.