When you're a new writer, just flushed with the first joy of seeing a vague story line morph into a real manuscript, you're looking for any help you can find in getting this book into print. That's why most bookstores have a shelf devoted to just those kinds of books: how to find an agent, how to get your book published, how to write a query letter, how to promote your book. . you name it, some other author has already written a book about it--and gotten it published!
The Complete Idiot's Guide to Getting Published pretty much says it all. This book takes you from writing to promoting. How to Get A Literary Agent is more specific, but it contains pages of good information on why you need a great agent and how to go about getting one.
And of course, don't forget the on-line sources. Just search literary agents, how to get my book published or anything along those lines and see the hundreds of web pages out there.
The problem with all of this information is that it doesn't all agree. Each source has a different take on writing query letters, for instance. One instructs writers to be very business-like and formal, while another offers casual, off-handed letters as examples of a successful query. And each swears that their letter is the only one that will open that elusive agent door.
Well, they might not all be right, but each book has at least some good advice. And one time or another, one of those letters is going to be the right one with the right agent. At least I hope so. If not, I can always write another book: How to Write a Million Query Letters and Still Remain Unpublished.