I've just submitted the latest chapter of Yelka. Earlier in the story I mentioned 'Stasia' as a character without further explanation. In fact, she appears in 'Nadiya' in the Serendipity series as a rescued sex slave. I may repost the relevant chapter when I have time, but most of my readers seem to devour everything and are familiar with each different series. Of course I do cross over and bring characters in because they are living in the same universe. Perhaps I ought to include more explanation?
Ted