The Acorus is the taller, iris-like plant in the back. These are growing in our bog. This is the variegated form, though I'm sure the book (Using Plants for Healing - Rodale Press) is talking about the plain one which I don't grow.
The earliest mention that the book knows about are 4 references in the Old Testament, so this has been considered a medicinal herb for a long time. The roots of the plant were used in commerce in the Near East at least 4 thousand years ago. There are numerous references to its use by Native Americans who seemed to rate SweetFlag as good for just about everything.
Although this plant resembles an Iris, it is actually more closely related to Jack-in-the-Pulpit and Skunk Cabbage and is an Arum. The flower is definitely unusual as it is a spadix, a think spike that has minute yellow flowers near the top. It just looks like color until you get close as these are really, really tiny flowers.
Dried foliage was at one time sold to churches for scattering over the floors to provide a "saintly odor", or more likely, to cover up the smells of unwashed bodies.
As with other members of the Arum family, the plant grows always in swampy of stream-edge spots throughout the United States. The part used medicinally is the root, which should not be peeled, for the vital principles are just under the surface layer. The dried root is powdered and infused in water. There is also some mention of chewing the raw root to stop 'stomach rumblings'.
We have it growing in boggy places where it multiplies faster than rabbits, so we always have plenty to sell, trade or just give away. It will also grow in just a plain garden bed if you provide sufficient water. Just expect a smaller plant. It likes sun, but will also grow in light shade.