Copy it back to the main drive from any backup you have 10.9 on. If you don't have a backup, then you can reinstall the OS (the long way around since you only need to restore the one app), or extract it from the Mavericks installer.
If you don't have the 5.3 GB Mavericks installer, you'll have to download it again from the App Store. When it's done downloading, it will want to install. Press Command+Q to stop it. The download will be in the Applications folder. Copy it to any external drive to save it so you don't have to download it again.
Right click on the installer and choose "Show Package Contents". Double click the folder "Contents", then "SharedSupport". Double click the "InstallESD.dmg" disk image to open it. Double click the folder "Packages" that appears when the disk image is on the desktop.
Download Pacifist. Drag and drop the package "Essentials.pkg" onto Pacifist's interface. In Pacifist, expand the "Applications" folder. Highlight FontBook.app and click the Install button at the upper left.