A recent query in my inbox laid claim to being "the first" of a certain kind of memoir. As it happened, I knew that was not the case. I wrote back drawing her attention to the earlier book. As you might imagine, the querier did not fall on this information with effusive thanks, return emails of kitten pictures or even silence. Oh no, unasked for advice, particularly of the unwelcome sort generally get replies steeped in sulfur and singed at the edges.
The problem here of course is that if I know about the earlier book, it's a good chance that most other agents will too. And a quick search of the Amazon data base turns it up as well.
When you claim to be first or only, and I'm interested in your book, I dig around before I reply "yes, please send me your manuscript."
It's not so much it's a problem that you're NOT first, as that you are clearly sloppy in your thinking and research. Frankly, that's death for me in non-fiction. Non-fiction requires meticulous research and documentation.
I remember hearing the utterly amazing Robert Caro speak several years back and he just casually mentioned he'd checked with the Historian of the Senate six different times on a single fact, as he got more information about an event. I would have stood up and screamed "that's how it's done" as if he'd hit a home run at Yankee Stadium, but we were in a library and librarians always have me on my best behavior.
So, what does this mean for you in your queries and writing?
Obviously it means do your research. If you can't find books in your category, are you using the right category? And are you skimming rather than digging deep? And have you gone to your local library and found the reference librarian and asked for help?
If you're not sure you're the first or only, don't say you are. Find another aspect of your story that distinguishes you from the pack.