Excellent.
Thanks.
I'm going to go with only having one thing, and that thing being the next book. And I think I'll make it a direct link to the book's amazon page, and not to my own book site, as I had before. The idea was that the amazon link might change, and then I'd have to edit the backmatter. But I'd rather get that one click going to a page where people can buy the book.
Now I need to decide whether I'll include the beginning of the next book, or just the link.
I'll only use the mailing list call to action in books that are part of an unfinished series, where the sequel hasn't been published yet, and I don't have any like that at the moment. That seems to fit, to me. If the next book exists, call to action = link to next book. If it is still not published, call to action = new release mailing list.