One of the many things that irritates me about the publishing world is that agents invariably tell you that when you prepare a proposal for a nonfiction book, you must include the titles of at least three recently published books that are similar to the one you're proposing. When I first saw this, some years ago now, I assumed it was to reassure publishers that your idea was original. But I soon learned that the opposite was the case: publishers are as insecure as a herd of sheep--or at least agents think they are. They want to be assured that other publishers have found your topic suitable enough to take the risk of publishing it.
So the other day I was pleased to get a rejection from a publisher to whom I'd sent a proposal. (As you probably know, it's extremely difficult to find a publisher who will even consider direct submissions from that lowest of creatures, the writer. Publishers want to be assured that some person calling himself or herself an agent has already approved of the work.)
Anyway this publisher replied quickly and praised the idea, but said he was turning it down because "there's not enough here that's new."
Wow. So I immediately sent him a proposal for a biography I've been working on for some time, but about which I knew that one of the my former agents would tell me, "Nobody has ever heard of this person." That's another rule: you can't write about anything that readers are likely to find unusual or different.
So we'll see what the publisher's reply is to this.