It's true! Publishers do hire writers to ghostwrite books. These opportunities often center around a series or famous authors, and the ghostwriter usually doesn't get a byline or credit for these works.
There are three steps to getting freelance writing jobs in ghostwriting/work for hire fields.
First, educate yourself. One of the best articles I've ever seen that covers this subject is Jenna Glatzer's article on book packaging at right-writing.com.
Then, think through the opportunity. Without the byline, you may end up with nothing on the other side of the project except a paycheck. This is certainly OK, but make sure you are willing to work without a clip to show for it.
Go to where the jobs are. I have indeed seen ghost writing jobs posted at freelance writing job boards. However, they may be few and far between, and there's likely much competition. Instead, consider targeting publishers with a letter of introduction or some cold calling. You can find publishers through their associations like the American Book Packagers Association or the American Association of Publishers.
Like most freelance writing jobs, getting ghostwriting jobs is a matter of finding the postings, cold calling the companies, and presenting yourself as an attractive freelance writing package, with a resume, clips and references.