Stir together tomatoes, eggplant, zucchini, celery, onion, tomato puree, vinegar, sugar, parsley and basil in a slow cooker. Cover and cook on HIGH for 4 hours or LOW for 6 hours.
Stir in olives and capers. Serve warm, at room temperature, or chill overnight. Spoon about 1/4 cup onto each toasted bread slice.