Unfortunately, the answer is: "It depends."
IF you are already well known in your field (a known actor, a pro
athlete, someone we see on tv fairly often), then YES, it is better to
approach a good literary agent, rather than market your book directly
to a publisher. This may also be true if you've written at least one
book in the past, through a traditional publishing house (i.e., the
book wasn't self published or published through a very small or
university press). It isn't that a publisher wouldn't be interested in
your work; it's just that an agent should be able to get you a better
contract and a larger advance.
On the other hand, if you aren't well known and you've never written a
book that was published traditionally, it's doubtful a "high end"
literary agent will be interested in your project. In fact, it's
doubtful that any good agent will be interested in your work. In this
case, it's much better to work up a great query letter to send
directly to publishing houses. (Only send in your already-prepared
proposal if an editor requests it after viewing your query letter.)
Not all publishing houses will look at unagented material, so it's
important to do your homework first. Writers Market is a traditional
and good source for such information ( www.writersmarket.com ).
As for your proposal, I highly recommend the book "The Shortest
Distance Between You and a Published Book" by Susan Page (found at
Amazon, or any good book store:
Here is a pretty decent example of a book proposal:
http://www.klance.com/Book_Proposal.htm (Kathryn Lance, Book
Proposals) Every book proposal should contain the following:
* A title page (title and author's name centered on page, author's
contact info in the lower left corner)
* Contents of proposal
* An overview that hooks the editor into the idea, piquing their interest.
* Author bio (why you are the best writer for this book)
* An examination of who the book's target audience is (including
* An examination of the competition. How is your book different and
better than other books currently available on the topic? (Don't
simply say "there is no other book like this.")
* A section on how you will promote the book.
* Three sample chapters.
This is all assuming you're writing a nonfiction book. If it's
fiction, check out "The Fiction Proposal:"
Researcher's personal knowledge
Google search for: "Sample Book Proposal," "Fiction Book Proposal"