I don't know how it works in the US, but in the UK, your tax-free allowance applies to your entire income, not each part of it separately. So if you have used up your allowance in your "day job", you will have to pay tax on the $10 you make from your book sales
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As you may know, I write and sell software, and although I don't make a vast amount of money from it, every penny that I do make, I have to pay tax on, because my tax-free allowance is used up by my salary from my day job.