The problem with most herbal medicines is that they don't do anything and aren't based in any kind of science. You might see a book claiming that "the celts used it to treat scrofula" or whatever, but that doesn't mean that it actually does stuff. There is a persistent belief that old timey herbalists knew how to treat all sorts of maladies by collecting herbs, but it's just a classic example of the "appeal to the ancients" logical fallacy and the "appeal to nature" fallacy combined with a decent amount of "noble savage".
Going back a couple hundred years, outcomes of a person visiting an herbalist and a person visiting a mainstream physician would potentially be in the favor of the herbalist cause doing nothing was better than whatever the mainstream physician was doing.
That all said, there definitely are plant/animal/fungi based things that do have real physiological effects. The problem is that dosing is difficult. Aspirin, aka acetylsalicilic acid, can be made easily from salicylic acid and acetic anhydride. The salicylic acid is the part that has effects, but it can be hard on your stomach by itself. Salicylic acid (or compounds that metabolize to it) can be obtained from willow bark, some birches, and wintergreen. Wintergreen itself can be a little bit of a topical analgesic (methyl-salicilate in it is an ingredient in bengay).
Quinine really helps malaria.
Opium obviously works, but maybe dont make it.
Some plants that people used historically to treat constipation or to staunch bleeding will probably work cause it's pretty obvious if they do.
There are books and other resources on evidence based herbal medicines, but it's really important to consider the type of evidence. There are a million studies showing certain compounds have potential effects on certain processes, or real effects shown in vitro, but it all gets beaten by randomized, controlled, trials (and ideally, meta-analyses).
Lastly, keep in mind that herbal remedies are often portrayed as low-risk, but that shouldn't be the case, either because there are plenty of ways for them to have negative effects or interfere with other medicines.