It will be hard to find medicinal, ethnobotanical qualities in Plant I.D. Field Guides in general.
What you're looking for are herbal medicinal books, remedial herb books, or local ethnobotanical books. These will cover what Kevin said are usually labelled 'weeds,' which is just a socially constructed concept.... Most wild weeds/shrubs that are found everywhere do have useful qualities. That's why dandelion is everywhere, the indigenous peoples brought them with them!
Even Bermuda Grass, used in India for millenia, increases the number of red blood cells in our body. It contains protein, fiber, calcium and phosphorus, potassium etc. It is an excellent detoxifier. It helps in maintaining the alkalinity of blood. Bermuda grass is used to cure diseases. What do we do with it? How do we use it? It is purely societal.
The potency of the average 'weed' is amazing simply because we haven't cultivated, hybridized and GMO'd it into oblivion, like other plants. You can't say a tree/shrub/weed has no qualities or uses.
Even the common Dahlia, considered and ornamental now, was once a food crop. The flower petals are edible salad greens and the tubers, in the Yacon family, were utilized by the peoples of North and South America before it was considered attractive and turned into a nursery plant and consequently hybridized.
An excellent book on Medicinal Plants and Weeds is by
Penelope Ody, 'Complete Medicinal Herbal', a must have with color pics and all.... I mostly utilize material on the plants and materials of local native peoples... But again, cross-reference always!
Make sure the author knows what they are saying. If they use terms like: '...This plant is said to..." this means the author has never tried it and really has no idea...
In learning the use of a plant, chose one and learn it completely.
At the end of a year, know five inside and out.
You will find that plants like Dandelion and Plantain are so potent and multi-useful that knowing between 5-10 plants will get you far, and cover most all of the bases.
Being an Encyclopedia and knowing 500 is often more confusing before it gets easier! Trust me I'm a botanist!
