I think it makes sense that the extra visibility from your series would move your first book. Have I noticed it in my experience? No
. I've had a steady sale of my first book daily and it's salability has not changed. What I thought was going to happen was a drop in sales of the first. But this hasn't happened either, fortunately.
Now the strangest thing, which I commented here before, is that I've had a surge in my
series books all selling at once. I can't be sure it's from the same buyers, but typically the trend matches three purchases at the same time. And my box set is four dollars less (?). But this might explain what's happening to you. Readers are seeing the boxset more like an advertisement and then separately buying your books.
But all this is my
Perhaps some writers out there with professional non-book selling marketing experience might know better. Writing is the first time in my life that I've had to learn how to sell things directly.
(edited for clarity)
