The September 2010 coupon book is out and you can download and view it right here on Costco Insider. Coupons are valid September 16th to October 10th. You do not need to bring it to the store to get the discounts, just mention it to the cashier.
Costco Coupon Book 9/16 to 10/10/10 (September 2010)
September 11th, 2010 Insider