Can I sign a W9 under a pen name?

I understand your concern as a fellow author using a "pen name". Nevertheless, online one of the W – 9 you are required to put your real name. Online to you can put your pen name as your business name.
I hope this helps and good luck with your writing,
Ron Cappuccio

I don't understand why you're asking your client's to fill out W9's and to whom you "turn in" those forms.
A W9 is used by a hiring party to learn the Social Security number of the person hired so the hiring party can notify the IRS of the amount. The form is not turned in to anyone but, rather, is kept in the hiring party's files. What is turned in is the 1099 reflecting the amount paid.
I understand you saying that YOU are the writer and, I assume, you get paid for your writing. You're the one who should be filling out W9's for those who pay you. If you pay researchers, editors, or anyone else who helps you put out your writing products then you need those person's to fill out W9's.
Please speak with a California-licensed tax attorney or accountant.

You will need to use your real name when filling out the W-9s.