I found the book faster than I thought. There was nothing specific about inside corners but it did give instructions for scallops. You sew to about 1/4 out then stick a pin in at the angle you want and then it looks like you fold it back over the pin. I have only done it once. It took some work but I got it.
Another possibility, depending on how picky you are, is to fake the miter out like you can do when you are putting on rick rack etc. One piece goes straight, then you iron a folded seam into the other and put the first one inside. Then tack it with stitching or glue it.