Simple, two options.
- You can point to a redirect-URL after form has been submitted to a special page.
See form options:
Redirect for example to:
Where the “hidden” is a folder and it has random pages. That page you can insert the link to the ebook.
Option 2:
Host the files on dropbox, make the end of the link go dl=1, and people download it automatically if you insert that link in the redirect.
See:
Disadvantage: people are stuck on the form page after that.