Can I use my Author voice for some books and not for others?

Yes, from version 1.9.18. Each author profile has its own voice, so a pen name for thrillers can write differently from the one for business books. On a book's page, the "Voice" card shows a line "Author voice: on" when the book's profile has a voice, with a switch. Turn it off and that one book is written only with the style and tone you chose; your profile and your other books are not affected. To stop using a voice everywhere without deleting it, open Authors, open the profile, and click "In use" at the top right of the "Author voice" card: it changes to "Switched off".