The download button on my book is grayed out and I can't click it.
If you've recently made an edit to your book, it could take a few minutes for the download file to render. Try refreshing the page and checking back in a few minutes to see if the button is clickable.
If it's been more than a few minutes and the download button is still grayed out, your book could be in a "stuck" state. To fix this, open your book, make a minor edit to it (add a space or a period somewhere), and then save and exit. This will reset the rendering process, and your book should be available for download within a few minutes.