Thin, yes, I have almost all of them. And the good news is that every one is a great book. There are one or two sitting in my Amazon cart to buy next, but here are my favorites:
The one I use as my base chocolate is in, Whoopie Pies, by Sarah Billingsly and Amy Treadwell. To me this is perfect. It is not overly sweet, but with the filling, it is perfect. The filling does contain vegetable shortening, which I hate, but to make it authentic, it has to be there. The marshmallow fluff would be too sticky for kids to enjoy. I try to stick with authenitcity, but if you want something similar without the shortening, an Italian meringue will be just as good, if not better in my opinion. It will taste like marshmallows. Just look up a recipe. It's essentially the same as IMBC with no butter. In this book, I refer to their other recipes to look for balance, but I have made my own for pumpkin and red velvet.
My next favorite is, The Whoopie Pie Book, byClaire Ptak. I saw this book months before it was released and it prompted me to experiment with gourmet whoopies. I am opening a retail shop soon and these will be one of the features. Again, I rarely copy a recipe, but this book has been an inspiration in just how gourmet these things can actually get.
By the way, I have experimented with every size and I think the #40 scoop, 1 1/2 tbsp, is a great single serving size. They are about 3 inches.