This is a tough question to answer because at some level you just need to trust that publishers and agents are going to read and evaluate your work without ripping it off or otherwise stealing your intellectual property. Certainly you can and should copyright it.
The simple solution is to have an explicit copyright and "all rights reserved" on every page, like this:
© 2007 by Dave Taylor. All Rights Reserved.
You can also use "(C)" or "(c)" as your copyright symbol, but don't do anything goofy here, just a simple, plain straightforward copyright is best.
If you want to protect yourself more, you can apply to the US Copyright Office for a formal copyright, which will require that you submit a copy of the completed work to the copyright office, but does offer stronger protection in the eyes of the law.
Nonetheless, both of these are of limited value because precious few ideas are truly unique, and just because you sent in a collection of personal anecdotes and 18 months later the publisher came out with a different collection of personal anecdotes that seems to have a half-dozen stolen from you, doesn't mean that's what happened. For one thing, they probably saw fifty other proposals or manuscripts on the same topic, but more importantly, it's quite unlikely that the other author even saw a page of your own submitted manuscript.
Generally, publishers want to work with authors through agents for this, and many other reasons, and your agent will be able to advise you on how to protect your own intellectual property and how to find the balance between that protection and actually getting it published. Typically an agent works for a percentage of the value of the book (15% is typical) and any agent that requires a fee to review your manuscript should be avoided.
If your book is accepted then the publisher will often pay you an advance against future book sales royalties, but that advance is almost always paid in stages as you meet specific schedule milestones for the work. For example, you might get a $5000 advance, paid 25% on signing the contract, 25% on delivering the first half of the work, 25% on delivering the last chapter, and 25% on completing the edits and changes suggested by your development and copy editors.
Oh, that reminds me that just because you have what you think is a completed manuscript, it doesn't mean that a publisher would read it and say "let's go to press!". Much more likely is that they'll like the concept, the idea, the voice, etc., but still require that you make changes, edits and updates, often substantial ones, before it's ready for publication.
Anyway, hope that helps you figure this out. There are a number of books in the library and bookstores on how to publish your own book - it might be worth an hour to sit and read through one of those too. They'll have lots of good information on how to pick an agent and so on.
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In copyright law, as soon as you have written something down (and you can proove the date) on any material (a napkin would qualify), it is copyrighted and you are protected against intellectual theft.