Up until very recently this versatile Mexican tuber was not well known outside of it's native range, except in Asia, where it has been popular for centuries, owing to it's early introduction there by way of the Philippines, again because of those Spanish galleons out of Acapulco. In many Asian countries, along with a few other New World plants in the Pachyrhizus family, is often called a "Yam Bean."Though in the Philippines it is still mostly referred to as "Jicama," another Spanish corruption of a Nahuatl word Xicamatl (pronounced Shee-cah-mat(l), or the indigenous Tagalog corruption of that word singkamas. It is also sometimes called a "Mexican turnip." In fact, the largest jicama ever to be harvested was said to have been grown in the Philippines 2010, it apparently weighed in at 23 kilos.
The reason for the name "yam bean," is because all of the plants in the family not only produce edible tubers of various sizes that are most prized for food, the so-called "yam" part of the plant. They also produce beans in papery looking pods, that are, like the rest of the plant save the tuber, is very poisonous. This classifies the plant as a member of the legume family....weirdly like peanuts!
In recent years Jicama has become much more well known and much easier to find in the everyday supermarket produce scene. They keep well at room temperature and thus, easily transported. So more and more markets have them in climates where they can't be grown outside of green houses. Now debating the merits, or mostly lack there of, of long range food transport, the fact is that most Americans now at least know what this vegetable looks like, even if they don't know what it is or what to do with it because of such transport. Having said all of that, let me also say: they are super easy to grow at home!
Like sunroot (aka sunchokes), with whom they are sometimes prepared, jicamas get their uncharacteristically sweet flavor from inulin, which makes them another great food for people with diabetes. As with sunroot, this compound is not a "net neutral" food when it comes to blood sugar levels, it has been proven to actually aid in naturally lowering blood sugar levels. It is also low in calories, which makes it an ideal "diet" food, since it does have such a sweet taste and it filling. Nutritionally, it is extremely high in dietary fiber that is easily pushed through the digestive system. They have trace amounts of protein and lipids. It should be noted, that Inulin is prebiotic, meaning that it can't be digested, and just like in sunroot, this can cause some people to have gastric gaseous build up until they get used to consuming the tuber. This is not nearly as pronounced as it is in sunroot, and only the most sensitive people have this when consuming jicama.
As to what to do with them, well an extremely ancient way to consume them is raw with or without powdered chile and/or salt. In post-contact times, a squeeze of lime has been added. In Mexico, a very popular lime/chile spice mixture called tajin, which is named, in Tontonac, for the ancient "pyramid" (Teocalli in Nahuatl) complex in their native Veracruz they call El Tajin. It is often also put into salad. The Yucatec (a Maya people of southern Mexico) are fond of tossing it with oranges and cilantro and dressed with red hot chili pequin, a mixture they sometimes call "Pico de Gallo" or rooster's beak in Spanish (not to be confused with the popular national salsa of the same name). In mercados, they are sold on sticks with seasonings for snacks. Some salads pair them up with Sunroot. They are also popular vegetable dippers (crudites), alone or with other vegetables. Grated they make a great slaw. In the Philippines they are popular in the fresh vegetarian native Lumpiang (Lumpia).
Cooked it goes into mashes, soups, sautés and stir-frys. Filipinos are fond of serving it with the native Bagoong--as shrimp paste. In southeast Asia it goes into various types of spring rolls and egg rolls, both fresh and fried. They can be part of a tempura platter, and, like regular turnips, they seem like they are ideally suited to carving. In Muslim area, they used as a stuffing for wheat flour rolls during Ramadan. It has also become popular to serve it with hummus. I north American native circles, Jicama has become a popular native food, even showing up in various incarnations at Powwow food vendor stalls. For example, they make an interesting additions to Indian Taco toppings.