The stevia plant is a small green herb, and its leaves are extremely sweet. The leaves are ground into a fine white powder. If you are in the US, it is only stocked in the supplement section of the health food store because the FDA has ruled it as unsafe as an additive but not unsafe as a supplement.
Make sure the brand you use is pure stevia; otherwise some of the recipes in my cookbook will not work. Some of the brands in the supermarket have a huge label that says ‘Stevia’ but it is actually mixed with something else to bulk it up. Stevia itself is ultra sweet, so it is only used in small amounts. So, if one of these alternative brands is used, you could imagine that the result would not be very sweet.
The brand I use and reference in the recipes is Nirvana.