A good quick read
I don’t use every plant he discusses in this short article. However, based on what knowledge I have, I feel the authors plant choices are a good representation of ‘the first 20 plants to learn.’ I could argue over a few of them (dandelion can be found in many cities and forests but his location in Canada might not have a lot more of this plant).
For two dollars this one is worth a purchase. For more depth and better pictures, you should purchase books by John Kallas and Samuel Thayer.