As I said, it's been a long while since I read them & I probably should re-read Perelandra. I have a feeling I liked it, but had some reservations & then went off it retrospectively after being forced to read Milton! I've not read any Charles Williams, but I agree that the change of tone in THS really didn't come off. The other two were science-fiction; that one wasn't & at the time left me with a bad taste in my mouth. There were open nods to Tolkien, too and that didn't feel right, either. If it hadn't been connected to the others, maybe it would have worked? I'm going to have to reread them now...