There is a strange phenomenon I have encountered in the course of being an editor, both in-house and freelance. The first time I came across it, I was dumbfounded by it, and it has become no less mystifying to me since then.
What is this baffling occurrence? It's when authors send a manuscript for consideration along with a strongly worded letter warning the recipient against stealing the book.
I once even had an author insist on a face-to-face meeting (another big no-no) who claimed that another publishing company she'd submitted her work to had come out with a book very similiar to hers right after she submitted her book to them. She was very distrustful and reluctant to leave anything with me to look at because she was afraid we might steal her work too.
I can assure you that the other publishing company coming out with the book similar to hers was purely coincidental. I know this because first of all, she said they came out with the book a couple of months after she submitted her manuscript, and publishing companies work much further in advance than that. Most publishing companies have their schedules lined out a couple of years in advance, and there's no way a book would be ready to publish only two months after it was submitted. Second, after looking at her books, I can tell you with confidence that they were not ready to be published. She was a new author, and her writing was not quite up to par.
This phenomenon is confusing to me for a couple of reasons. First, an author presumably trusts that a publishing company is competent enough to edit, publish, and promote his or her book but doesn't trust them enough not to steal his or her work. That's just illogical. Second, most authors who have put any effort into learning about the process of publishing realize that publisher receive hundreds (and most of the time thousands) of unsolicited queries every year. They are not hard up for books to publish. If a publisher thinks your work is good enough to publish, you'll probably get published. If the publisher doesn't thinks so, it'll just pass on the work and move on. The likelihood of a publisher stealing your work is extremely slim.
By sending such a warning, you're starting off on the wrong foot with a publisher (or an agent, for that matter). It screams, "High maintenance! Hard to work with! Prima donna!" Trust me, this is not the message you want to send to a publisher you want to work with.
So, you might ask, how can I protect myself from those rogue publishers and agents who are out there stealing authors' books willy-nilly? First of all, relax. I can say with a lot of confidence that this is not going to happen. Agents and publishers are interested in promoting authors and publishing books, not plagiarizing their material. If you must, you can mail your work to yourself in a sealed envelope. Having a date stamp on the material will show without a doubt that you are the author of your work by a certain date. It really isn't necessary to apply for a copyright until your work is being published, although you may also do that if it will make you feel better.
But most of all, just take a deep breath, calm down, and realize that publishers and agents aren't out to get you (although it might feel like it after a few rejections). There are many other things you can spend your time and energy concentrating on, like revising your manuscript so it's the best it can be and writing a killer query letter.