Sales figures
In a query letter, sales figures and comparisons can sometimes make an agent or editor think you've done your homework, and agents and editors do appreciate writers who do their homework. But no one is really going to care about the numbers you cite or the comparisons you make until after they read the manuscript.
And when reading a manuscript, agents and editors are going to make their own comparisons.
Any editor who works at a conglomerate publisher can get nearly perfect numbers instantly, and so can any national distributor or chain bookstore. Chain bookstores and national distributors make many of their decisions about books by checking the numbers before doing anything else. They know exactly how many copies of your last novel sold. Neilsen is just one tool they use. If national distributors and chain bookstores didn't know the numbers without the writer telling them, they wouldn't last long. By and large, national distributors and chain bookstores always use the numbers they have access to, which outstrip Nielsen from their point of view, to determine exactly how many copies of a writers next novel to handle/stock.
And any experienced editor can make a phone call and get nearly exact numbers, even if they don't have direct access to Nielsen.
And any editor has exact numbers for the books his own publisher sells, of course. They couldn't pay writers without knowing how many books sold. And when this editor works for a conglomerate, this means access to nearly exact numbers for all publishers that fall under the conglomerate, and for all the booklines at each publisher.
But in truth, exact numbers are far more imortant to national distributors and chain bookstores, both of which already know them, than they are to agents and editors.
An agent or editor generally reads a manuscript and likes it or not. If they like the manuscript, sales are judged, accurately, by thoughts such as "Books like this generally don't sell well, generally sell moderately well, usually sell pretty well, often sell extremely well."
And comparisons in fiction are often risky. If you say, "My novel is like a cross between Lawrence Block and Robert B. Parker," an agent or editor is always going to be tempted to read a few pages and say, "Like hell it is!" Not to say a writer shouldn;t make compartisons to other writers. They can be helpful. But they had better be apt comparisons, and when dealing with agents, it's best to compare your novels with others that particular agent has handled.
Knowing sales numbers for your own book shouldn't require access to Nielsen, and in all truth, it's no one's business outside of publishing what the sales numbers are for someone else's book. Why not just go to the writer and ask to see his royalty statement or his checkbook?
If you sell a novel, it will be because an agent or editor thinks it's a novel the public will buy, and they won't use any numbers or comparisons the writer makes to reach this decision. And trying to write a novel in a certain genre or because a given novel had great sales is almost always self-defeating. To write well, you have to write what you love to write, not what someone else had a bestseller with.
