Always submit your book manuscript to a legitimate publisher. You can find legitimate publishers in the latest edition of Writer's Market or by subscribing to Writer's Market online. Also, there are a number of legitimate market listings available online that not only list markets for your work but also weed out the scam artists as well (see resources).
Be suspicious of publishers who say they can guarantee publication of your book, especially if this guarantee is made before seeing the manuscript. This is a red flag. Publishers can't guarantee you publication before they've read the manuscript, and if they do, chances are good that they want to sell you something.
Beware of publishers who promise to publish your book with a "little editorial polish" and then ask you to pay for their book-doctor services. Book doctors are legitimate, but the author would be the one to contact and use a book doctor. If publishers ask you to use their service, they aren't interested in publishing your manuscript. Their goal is to sell you their editing services.