Just because someone else has already written a book similar to yours doesn't mean you don't have a salable idea. But agents and editors will want to know how your book differs from those already on the market. What you need to know, and what you need to tell agents and editors, varies according to the kind of book you're writing.

Fiction Research

For fiction, it's most helpful to point out books that are similar to yours; if your story is reminiscent of Tony Hillerman, for example, an agent or editor will start thinking of ways to tap into that vein of readers. Go to your local bookstore and see which books are being aggressively marketed with front-table displays, posters, and so on. Make a note of the authors, titles, publishers, and genres of these books, and check out the acknowledgments for agents' and editors' names; this will help you target potential agents and editors for your book.

Do I need to find out how many copies of a competing book were sold?

No. Publishers have access to much more accurate copies-sold data than you can find, and that information is an integral part of their decision to purchase a manuscript. However, if a similar title has made the bestseller list, point that out in your cover letter or proposal.

Nonfiction Research

Nonfiction books need to stand out from the crowd in order to catch an agent's or editor's eye. No matter what your topic is, chances are there's a similar book out there somewhere. You need to find out what those titles are, what their focus is, and who is likely to be most interested in them. You may have a great idea for a book on herb gardens or the history of the potato chip, but until you know what other books have to say on those topics, you can't fine-tune your idea.

It's important to note here that your idea of a competing book may not be the same as an agent's or editor's idea of a competing book. Let's say you want to write a complete guide for babysitters about how to keep children safe and what to do in various emergencies. Maybe there isn't another book quite like this on the market. But perhaps there is a babysitting book that covers this material in one of the chapters. An editor may consider that a competing book, especially if that book is published by his own house. If the existing book is selling well, the editor may be interested in yours to be marketed as a companion guide; on the other hand, the publisher may not want any new titles that could siphon sales from the existing book. If the existing book is not selling well, chances are the house will be leery of taking on a similar title, unless you can convince the editor that yours will do better.

Online booksellers like Amazon.com are excellent sources of information about books in print. Even upcoming books — those to be published within the next four to six months — are often listed on these sites. You can find out the publisher, the format, and often even the table of contents, or maybe read excerpts of competing books.

Amazon.com and other online booksellers usually list sales rankings for titles. For your purposes, these rankings are irrelevant because they only reflect sales on that particular site. Many authors assume the rankings show how many copies a particular title has sold, but that's not true: The No. 1 seller on Amazon.com may have sold 1,000 copies, while No. 3,642 sold 910 copies. Your best bet is to ignore the sales ranks altogether.