Everybody who follows the guidelines for a book proposal knows they are supposed to include something about complementary and competitive titles. You'd think that writers would want to study up on the subject, go to Google and search for "competing titles" or "competitive titles" to learn more about what the agent or editor wants to see. After a quick experiment, I'm estimating that one or two authors do that each day, IN THE WHOLE WORLD. I think my correspondence gives me a little insight into why.
In the first place, authors are highly resistant to doing any homework at all. If people were as resistant to the common cold as authors are to studying up on the publishing industry, we'd call it the uncommon cold and persecute the people who got it. But when you get past the "I've written a bestseller, why should I waste any time on this stupidity" attitude, I think a lot of authors assume that it's a trick. As in, "These people just want a reason to reject my book, and if I admit to any successful competing titles they'll say it's been done already. Hah! I'll outsmart them. I'll say that the book is unique!"
Lack of research is a big part of why so many authors who can write nice prose fail to get published. Either they don't provide the information an agent or editor has asked for in a book proposal, or they write their special book without looking at the market first to see if there's any existing demand. Publishers really want to see competing titles on a book proposal. If there are competing titles that have strong sales, that's even better. If the competing titles are commercial failures and you can't give a persuasive argument as to why your book will do much better, it will hurt, but the publisher is going to find out anyway. Agents and editors will take two minutes to pop over to Amazon and drop some key words from your title or you query letter into the search box to see how related titles are doing. If you've written a book with little chance of commercial success, you aren't going to hide the fact from people who make a living selling books. I just searched my archives and found I've written two related articles about being your own acquisitions editor and self publishing as a business.
Trade publishers don't expect unpublished authors to have access to Neilsen BookScan to check title sales, but they expect you to be able to go to a bookstore and browse the shelves, and to visit Amazon. Amazon has these beautiful category lists to help you look into what is and isn't selling in very narrow niches, but make sure you use the ones near the bottom of the product page, under where it says "Look for Similar Items by Category." The lists right under the sales rank, where it says "Popular in this category" are updated every hour, so the results show wild variations. But the most important tool you can bring to competing titles research is your common sense. Read the competing books before you praise or trash them in the proposal, and you might learn something about the advantages, or flaws and omissions of your own book.
I'm planning on taking a three day break for the Jewish New Year, so forgive me if any comments linger in the queue even longer than usual:-)