Eggplant is a nutritional void; it's got precious few vitamins, it appears to be made mostly of cellulose, and it's slightly bitter, especially in older specimens. Why, then is it so popular?
Perhaps because it was very popular as a meat substitute, as a meal filler and because it grows abundantly on every continent. Eggplant will grow almost anywhere it can get enough water. It will tolerate saline, alkaline, clay-heavy and even volcanic soils; it will put up with rocks in its root system, and it's not terribly picky about whether it gets full sun or partial shade. Sure, there are ideal growing conditions, but if you don't have them, you can still grow eggplant.
When buying eggplant, look for smooth, unwrinkled skin. “Dirt spots”, where the eggplant rested on the ground, are fine, but moldy spots are not. To determine whether your eggplant is truly fresh, look at the cap of green stem on top; if it's started to brown and looks peaked, the eggplant it's attached to will be past its prime and unpalatably bitter.
The old story about eggplants being male and female (and male eggplants having fewer seeds, being less bitter and therefore superior) is chauvinistic bullcrap. Smaller, less mature eggplants have fewer and smaller seeds; “gender” is irrelevant, so stop looking at the bulb end of the fruit to see whether it's got a pipe or a flange, okay?
What kind of eggplant, though? The giant globe eggplants common in grocery stores are as bitter as gall but make fantastic slices for eggplant parmigiana. If you're going to make “fish-fragrance” eggplant (which contains ginger, garlic, hot bean paste and no fish whatsoever), however, you'll want the long, light purple Chinese eggplants. Stuffing them with ground meat, pine nuts, garlic, parsley and lemon zest and braising them in tomato sauce? Go for long, slender, dark purple Italian eggplant. Thai curries require the globe-shaped, small green Thai eggplants, variegated like decorative plant leaves, and Indian curry will go best with the egg-shaped, purple-and-white Indian eggplants. African eggplants are white or cream-colored and are the least bitter of the plants.
Incidentally, at Asian vendors you may find wild eggplant; these aren't true eggplants, but a close cousin. They are the size of peas and always, always sold attached to their briarlike branches. They are intensely bitter despite their size; use a salty sauce (preserved black beans go wonderfully) to cook these.
Eggplants, like tofu, will soak up whatever they're cooked in; they're vegetarian sponges. The problem is that they will also soak up their cooking medium, meaning that if you aren't attentive, you can end up with a greasy mess: one single globe eggplant, cut into inch-thick slices for parmigiana, can soak up half a liter of olive oil.
If you are using large eggplants, or any eggplant with prominent veins of seeds (this is an indicator of age), or if you're going to fry the eggplant, you should purge the fruit by cutting into the desired pieces, sprinkling heavily with salt, and leaving to drain in a basket suspended in the sink for an hour. Rinse the eggplant pieces and pat dry before using; this process leaches out some of the bitterness and also makes the eggplant soak up less oil during cooking.
The greatest variety of eggplants will be available through August. Take a new kind of eggplant you've never tried before and cook it with olive oil, onions, garlic and bell peppers. Add a tomato or two and some herbs and you'll have the Provençal sauce known as “pebronata”, which goes stunningly well with pasta and fresh goat cheese. Or pierce the eggplant a couple of times with a knife and set on a hot grill until the skin chars and starts to flake. Peel the eggplant, then mash it with tahini (sesame paste), lemon juice, parsley and finely-minced garlic; congratulations, you've made babaghannouj.