I changed computers and cannot find my book.

Books are stored in the browser on the computer where you made them, so they don't automatically appear on a new machine. To bring one over:

• If you saved a project file: copy it to the new computer and use My Books → Open project file… to restore the whole book. This is the intended way to move between computers — see Can I move a project to another computer?
• If you backed the book up with the ☁ button: sign in on the new computer and it's waiting in My Books. Open it and carry on.
• If you did neither, but you purchased: sign in on the new computer and your license keys sync back, so you won't pay again — though you may need to rebuild the project from your manuscript.
• If you did neither: the local project stayed on the old computer. Go back to it, download the project file, and move it over.

Going forward, a project file is your portable copy — keep one in a cloud drive.