My favorite resources are actually from the blogging community. A lot of my ideas and recipes come from blogs, especially the ones I have listed on the blogroll. But, a few of my other favorite resources are:
Cookbooks:
Baking: From My Home to Yours by Dorie Greenspan
Williams Sonoma: Essentials of Baking
Joy of Cooking
Better Homes and Gardens
The Southern Living Cookbook
The Sweet Melissa Baking Book by Melissa Murphy
The Secrets of Baking by Sherry Yard
Baking Illustrated
The Pastry Queen by Rebecca Rather
Websites:
Dinner & Dessert 
Where can I purchase these whoopie pies?