Meanwhile, cook spaghetti in a 6- to 8-quart pot of boiling
salted water until al dente. Reserve 1 cup cooking water, then drain spaghetti in a colander. Stir zest and salt into oil in skillet, then add spaghetti, parsley, and 1/2 cup reserved cooking water. Toss over moderately high heat until combined well, 30 seconds to 1 minute, adding enough reserved cooking water to keep spaghetti moist. Add half of garlic and toss. Remove from heat and toss pasta with remaining 1 1/2 tablespoons oil and salt and pepper to taste. Transfer to a serving bowl and sprinkle with remaining garlic.