You can use the coupon to extend your current membership another year if you wish.
There is no expiration date on the coupon with the D300 or CLS books, so probably the coupon in the D700 book will not have one either. I haven't seen any mention of expiration in Nikonian forums either. So there shouldn't be any problem in delaying you use of the coupon.
I have both the D300 and CLS books. Based on those I'll strongly recommend the D700 book.
Gary in SE Michigan, USA. Nikonians membership - My most important photographic investment, after the camera. D4, D810, D300 (720nm IR conversion), D90, F6, FM3a (black), FM2n (chrome) YashicaMat 124, Graflex Speed Graphic 4x5 My Nikonians Gallery & Our Chapter Gallery