How can I permanently delete a local project?

To remove one book, open My Books and use the remove/delete control on that book's card. It's deleted from this browser's storage.

To wipe everything — every project, plus your sign-in — use the "Erase everything on this device" button at the bottom of the account area on My Books. It's built for shared or public computers: it signs you out and clears all local BookPublish data in one go, after a confirmation.

Both are permanent and only affect this browser. If you might want the book later, download its project file first — deleting locally does not touch a project file you've already saved, and it doesn't cancel any purchase (your license keys come back when you sign in).