It's taken me a long time to understand something about why some books sell and others don't.
It's taken me having written novels and marketed more than 1,200 others via AuthorBuzz.com.
Sales don't always have anything to do with good or brilliant or original.
Sales are about appeal.
The marketability, the success of a book, ultimately rests with whether or not people will find the concept/characters/title/cover appealing.
It's in the eye of the beholder, but readers know it when they see it. And we see the results.
Bridges of Madison County, The Da Vinci Code, The Help and Sarah's Key have very little in common with each other. But they all had that certain indefinable something that appealed to readers.
Nora Roberts, Stephen King, Lee Child and George R. Martin write wildly different books. Their writing, plotting and styles have little or nothing in common. But they all write books and characters that readers find appealing.
It's helped many writers I know, writers who sweat every word, who write truly magnificent books... when I explain this seemingly simple idea.
You can write the best book you can and that might still not be enough. Appeal isn't something that most writers can't strive for or identify. It's something even the best agents and editors can't always identify.
So beat yourself up over a whole lot of other things, but don't assume that because your book didn't take off, it wasn't good. Or even brilliant. It might just not have been appealing enough.