Some struggling airlines are offering bargains again, if you book your flights quickly. Northwest Airlines is offering discounts of up to 40% for travel from Nov. 24 through Jan.14 for tickets bought by this Friday. The tickets are nonrefundable and are good for travel only in the lower 48 states, Alaska and Canada. If you want to fly overseas, check out TWA, which is offering discounts of up to 30% on routes from 15 U.S. cities to 10 European destinations. The TWA tickets must be purchased by midnight tomorrow (Tuesday), and are good for travel through March 15, except during the holidays of Christmas and New Year's. Finally, Southwest Airlines is discounting 1-way tickets for travel within California between now and March 2, provided you buy your ticket at least 21 days in advance.