I’m guessing you can count on one hand the books you’re read more than once. We typically read for information or in order to be entertained by a particular story, and then we return the book or stick it on a shelf. If we really enjoy it we’ll recommend it to our friends.
If you read a book explaining the significance of the beaver trade in colonial America or telling the story of a young woman who finds out that she’s really the princess of a small country, I’m guessing you wouldn’t be tempted by another book that explains the animal-centered commerce of colonial America or how a country found it’s unsuspecting princess in a mall department store.
In other words, if you’re just rehashing what’s already out there, chances are you’ll have a hard time finding readers. That isn’t to say you need to have a completely fresh and unique idea that no one has ever done before. There are fresh angles to explore in topics that are already addressed in books and new spins we can add to old stories.
However, you need to watch out for the “been there, read that,” response from readers. It would be terrible to invest a year or two of your life into a book project that fails to sell because it’s been done.
I know that the spirits of aspiring authors are crushed every time a new vampire book is released, but even if there are plenty of repeats out there, publishers and the general public are looking for unique books with something new to say. For every vampire spin-off, there are plenty of innovative and unique books released each year. For example, I encourage you to read something by Susanna Clark, Jasper Fforde, or Neil Gaiman for examples of authors breaking new ground by tinkering with older forms.
Can you add a new angle to a topic that has been covered extensively? Can you draw in your readers without tossing in a character with a gun (such as secret agent Michael Scarn?)?
If someone has already nailed your book topic, I encourage you to buy that book, read it, and consider what else needs to be addressed in your own book. In fact, reading your competition is essential by way of not only selling your book, but making sure you write with an awareness of your genre and field.
If you claim to present a ground-breaking, fresh, new, riveting book that only rehashing what five others have already done, then you’ve just spent a lot of time working on the project that’s going no where fast.
Perhaps you could begin by asking yourself this question: What can I write that no one else can?