Put oven rack in middle position and preheat oven to 350°F.
Heat a dry 12-inch heavy skillet over high heat until hot.
Pat lamb dry and sprinkle meat all over with salt and pepper. Add oil to hot skillet, brown racks, in 2 batches if necessary, on all sides (not ends), about 10 minutes per batch.
Transfer racks to a small (13- by 9-inch) roasting pan.
Stir together garlic, herbs, salt, and oil. Coat meaty parts of lamb with herb mixture, pressing to help adhere. Cover exposed bones with foil and roast 15-25 minutes, until thermometer inserted diagonally into center of meat registers at your desired temperature. Let stand, covered, 10 minutes. (Internal temperature will rise while lamb stands.)
Temps after resting:
Bloody rare: 115 to 125 degrees F
Rare: 125 to 130 degrees F
Medium rare: 130 to 140 degrees F
Medium: 140 to 150 degrees F