It used to be that you had one chance to sell a book to any given publisher, agent, or customer. If the book you wrote wasn't the book they wanted, well, you took the battered manuscript out of the SASE envelope, sighed, and went back to Writer's Market to find another possible market. (Anyone else remember those days?)
Once in a great while, an editor would send a "revise and resubmit" letter, and you might get another chance... but usually that wouldn't work out as a sale either. Publishers often had pretty strict guidelines for genre books, and no matter what they said about wanting "fresh voices," they really wanted "more of the same, only better." Or they wanted something quite specific, or what they wanted one year they didn't want the next, and if you didn't hit the bull's-eye on just the right day, you were probably out of luck. (I speak from long experience of just barely missing many sales....)
But things are different now. The market has changed so rapidly, we're all scrambling to catch up. And the publishing industry, always so slow to learn from their mistakes, is at least becoming a bit more wary of wholesale and arbitrary rejection. No one wants to be like one of the nine supposedly smart editors who rejected the first Harry Potter book. They are starting -- some of them-- to look beyond and consider what might sell in the long run-- a wonderful series idea, a great imagined world, a compelling voice. An author with one or more of those might be worth talking to, even if the offered book isn't quite right.
Agents too are realizing that the easy sales they had gotten accustomed to aren't so easy anymore, and that their captive clients are feeling more liberated and expecting something more than just another sale to the same old place and the same old contract. Agents have always talked about longterm partnerships with their clients, and some even meant it-- but now that's getting to be a necessity. Authors have other options now, and agents are having to think of ways to make themselves useful, including helping to manage an independent publishing career. In this case, they also will need to consider more intangible aspects of an author's craft, including the ability to self-promote and use social media. Again, the book might not be "just right," but the author might be.
Finally, even those authors who have decided to forego the traditional route and don't have to hit the mark with publishers and agents still might have to face the most discerning of all critics, the reading public. Readers now are much more likely to choose an AUTHOR rather than a single book. (If they like the book, they want more from the author.) But readers can be capricious, turning away from a book because of a single word in the description, or because the book seems too dark or not dark enough or too derivative or too innovative, or… That's one reason they can be so loyal to authors they like, and why they are often willing to take another chance if the "problem" has been fixed.
Problem is -- authors have to find the readers/publishers/agents who will want these books... and sometimes that will require making major changes in a book with an agenda of getting that longterm relationship.
So the good news is: We're getting more second chances!
The bad news is: We have to take advantage of those second chances!
Let's talk about ways to take an existing book and reinvent it to take that second chance.