Tamarind by John Spottiswood.

The pulp from the tamarind pod is used as a souring agent in Latin America, India, the Middle East, Africa, and Asia. To extract the pulp, shell the pods, put them in a saucepan, then add enough water to completely cover the pulp. Simmer for about half an hour, then strain out and discard the seeds. It's a nuisance to do this, so many cooks simply buy the extracted pulp in bricks, jars, cans, powders, or bottles. There's also a sweet tamarind, which looks like the sour variety and is used primarily to make drinks.