Q: i seem to have deleted font book. how can i download this app back on?

It isn't in your Applications folder? Do you have a backup you could restore from? If not, you'll need to reinstall the OS.

You can custom install the Font Book directly from disc #1 that came with your computer when it was purchased.  Follow the onscreen custom install instructions.

If you’d like to reinstall applications that aren’t offered in the Custom Install screen—applications in the Utilities folder or in the Bundled Software folder for example—you can use Pacifist - http://www.charlessoft.com/