The simple solution is to set yourself up as a self-employed writer, filing on Schedule C. This is e-filed as part of your Form 1040 tax returns; no documents need to be mailed anywhere.
This will allow you to properly report all of your book sales as gross income. Additionally, you will be able to deduct all of your writing-related expenses, including office supplies, travel for research related to your book, advertising, and, perhaps most importantly if you are writing from your home, the Business Use of the Home deduction. This will allow you to deduct a portion of all the costs of your home, based on the area devoted to your writing business.
I am including links to some IRS publications that you may find useful: