Yep I definitely lost it somehow.
I checked in my trash too and it's not there either, so I guess I need to know how to restore it.
The easiest way will be if you've got access to another Mac that's also running Leopard. If so, you can simply burn that Mac's /Applications/Font Book to a CD and transfer it to your own Mac's /Applications folder with drag & drop. Follow that up by running the
Mac OS X 10.5.8 Combo Update if the source Mac is running an earlier version of OS X.
Alternatively (if you can lay your hands on your install/restore disc #1), you can run an
Archive & Install. (Be certain to check the "Preserve User and Network Settings" box if such is your intention and to
back up any data you can't afford to lose.) An A&I will leave you with the version of OS X with which your Mac shipped and
may need to be followed up with a running of the 10.5.8 Combo, but, under any circumstances, should be followed up with a check of Software Update to see which, if any, other updates you need to reinstall.
If neither of those alternatives works for you, or if you prefer a more technical approach, I'll walk you through a
Pacifist ($20 shareware, but free to try) extraction and installation.