What is a mid-list book? In fiction, according to me, the list works like this: Best selling authors on one end of the list (top) and classics (old and new) on the other end. The stuff in the middle is mid-list.
Everyone starts mid-list. Everyone.
The book and the author [each] have to prove themselves from there. No one knows for certain if a book will take off. Adapting a screenplay from a book doesn't guarantee that either the movie or book will be a success. A celebrity or political figure can be photographed reading a particular. Still it is the regular people who read a mid-list book and love it, tell their friends and they love it. Suddenly sales rise and the book gets noticed - and other good things happen.
Sometimes a well known author will have a really bad book. (Well they can't all be best sellers!). What do they do? They grin or laugh about it and write another book.
There comes a time in the writing process when the story gets old (it has been through a dozen rewrites and read-throughs) and confidence lags. I've spent one weekend cutting 5,000 words from Kathryn's Beach because it all looked bad to me.
The giddiness and self-indulgence of the early draft writing days turns to a seasoned writer, an author. Like any artist, then they can see the true beauty of their work.