Spaghetti with Garlic

Peel garlic without crushing cloves and thinly slice lengthwise as evenly as possible (you should have about 1 cup; peel and slice more if necessary). Cook garlic in 4 tablespoons (1/4 cup) oil in a 12-inch nonstick skillet over moderately low heat, stirring occasionally, until just golden, 7 to 10 minutes. Transfer garlic with a slotted spoon to a bowl. (Garlic will crisp as it cools.) Add hot pepper flakes to skillet and cook, stirring, 1 minute, then remove skillet from heat.
Meanwhile, cook spaghetti in a 6- to 8-quart pot of boiling 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.