You can find the Cookbook in Ashenvale. The person who sells it is in a lodge overlooking a lake to the southeast of Astranaar. It does cost 1 gold, so be prepared to pay.
Just a little extra information - it's a Retreat that is a building just above Mystral Lake in Ashenvale. If you go Southeast from the eastern edge of Astranaar you can get to it. Some bears in the way. Here's another cooking thread that asked the same question and gives the location for the horde cooking book as well.