3. The plot deals with an entirely new problem. You can often pick detective novels up mid-series because each detective story is a self-contained plot. They start with a new question and then have to solve it, tidily, by the end of the book. Shades of Milk and Honey had a Jane Austen plot structure so the big question there was: Who is she going to marry? That’s no longer a question. Since I’ve sent [person A and B] to the Continent in 1815, the big question in Glamour in Glass is: What are they going to do when the Battle of Waterloo happens?
A lot of fantasy writers (myself included) write book two (or perhaps two and three) of their fantasy trilogy while waiting to hear back from publishers about the first book. The fundamental problem with this is that book one may never sell – and then you’re screwed.
Or perhaps, if you write very carefully, not.
PS Although I think it’s solid advice above, I also think she may have gone too far, since the first book is a romance. That technically makes the two books different genres.