What is ground red pepper?
I have the 75th anniversary edition of Joy of Cooking. I've now come across two recipes that call for "ground red pepper" (enchiladas and a dry rub). I've never heard of ground red pepper before and cannot find it at the store.
Is it ground red bell pepper or something hot like cayenne? What should I use as a substitute?
The term "ground red pepper" is ambiguous in English. Things sold under that name in the US have a wide range of heats, and likely are made from multiple varieties of peppers, though I'd say they're more commonly hot things like cayenne, or other varieties with somewhat less heat.
That said, don't worry about it too much in these cases. Anything from mild paprika to cayenne would likely fit within what the authors meant, and you're not going to harm a recipe by increasing or decreasing the heat, or by using a chile powder with a slightly different flavor. Just use something (or a blend) that provides the heat level you want, and you'll be fine.