Actually, what shadowwalker said would only be true if you set up a "C" Corporation. These days, most businesses set up S Corps or LLCs, where there is no "entity-level" tax, so you don't have to worry about being taxed twice. C Corps are only required for certain complex shareholder schemes, mulitple classes of stock, as well as a few things like excess net passive income rules that can terminate S status.
I did a search and found this thread--ironic that it was only started yesterday. I, too, am trying to figure out for my dad what is the best way to report income from his book sales.
Since net income (and hopefully there will be) is subject to self-employment tax under a sole proprietorship, I'm thinking that setting up a corporation (and paying the shareholder/writer a portion vs. all of the net income as wages) would save approx 15% on the remaining income not taxed as wages; i.e., taken as shareholder distributions. The IRS cautions S Corps not to abuse this "loophole" - that is, the shareholder who generates the income must receive a wage, but it doesn't have to be total net income, so there's a potential for self-employment tax savings there.
Additionally, limited liability provided by a corp or an LLC is always a good thing, as (I believe) Xelebes referred to above.
However, I'm curious if anyone knows: does the copyright have to be owned by the corp, since the corp would be the one reporting the income from the copyright?