Thanks for the quick reply, ResearchGuy. I live in CA, so it looks like I do need a license... Ugh, not looking forward to all that paperwork
It is not that bad. BOE has a form online, I believe, and helpful people who actually answer the phone. Keep good records, and the annual filing/payment is pretty simple.
Business license will cost you probably a couple of hundred dollars IF you need it (maybe less -- mine is now $50/year). Maybe you don't, but that gets into a legal question I am not qualified to answer. In my case, I knew before I started the business that I'd be selling services, so I went to City Hall and got a license (and for good measure joined the Chamber of Commerce, which would probably not serve your purposes).
You'll also have to file business income and expense info. on Schedule C of your federal income tax return. But as a writer, you'd have to do that anyway, and maybe already do so.
--Ken