Hi Scott,
Always measure on the floor. One advantage your oven has is that the heat is stored in your hearth bricks and continues to radiate up even after the fire is out. If your floor is still hot, you’re still cooking!
As for a good recipe book, I usually recommend Andrea Mugnaini’s The Art of Wood-Fired Cooking. Her family imports and installs wood-fired pizza ovens, and also runs a cooking program at their home in California. The good thing about this book is that when she describes preparing your oven for different kinds of cooking (pizza, roasting, baking), she is thoroughly descriptive and includes diagrams.
There are similar books, but most of them stop with great pictures and recipes, which is where Mugnaini picks up.