Interesting that you should mention The Moonstone Castle Mystery, because that is one of the ones I have! The others are The Secret of the Old Clock and The Mystery of the Fire Dragon. Of course, I had read Old Clock before I got it, since it's the first, but I think the other two were more recent ones at the time, and my "sources" didn't have those. I see that they are #s 40 and 38, respectively.