Besides work-for-hire agreements, the other method is the royalty contract. That means you agree on a percentage split with the author.
This isn't quite accurate, but it will give you an idea of how royalties work. Think of 10 percent of the retail cost of the book as the royalty—the amount paid to the author/writer. (In the Christian market, the percentage is a little higher, but it's based on the wholesale price of the book, and the writers and authors make less money.)
Experienced ghostwriters usually split the 10 percent royalty per book at 50-50. If an agent is involved, her 15 percent fee comes off first and you split the 85 percent evenly or 42.5 percent each.
Most royalty ghostwriters start at small amounts, such as 30 or 35 percent and work their way up to 50 percent.