Make the caponata: In a heavy skillet heat 2 tablespoons of the oil over moderately high heat until it hot; add the eggplant, stirring, for 3 to 5 minutes, or until it is tender, and transfer it to a bowl.
To the skillet add the remaining 2 tablespoons oil; add the onion and the celery and cook over moderate heat, stirring, for 5 minutes.
Add the olives, the capers, the vinegar, the sugar, the raisins, the pine nuts, and the tomatoes and cook the mixture, covered, stirring occasionally, for 5 to 10 minutes, or until it is cooked through and the celery is tender, and transfer it to the bowl.
Stir in the parsley, let the caponata cool, and chill it, covered,overnight. Season the caponata with salt and pepper.
Make the bruschetta: Preheat the grill. Grill the bread on an oiled rack for 1 minute on each side, brush the toasts on one side with the oil, and sprinkle them with salt to taste.
Top each toast generously with some of the caponata.