There are two common causes for Font Book crashing (or immediately quitting) upon launch.
The first problem is caused by a corrupt Font Collection file which causes an exception when Font Book tries to unarchive it. (You may also be unable to show the Font Panel in applications that use it). This problem is usually limited to a particular user account. To resolve it, go to your /Users/~/Library/FontCollections/ folder and remove any non-default .collection files there; you should be able to
keep the following:
Chinese.collection
Classic.collection
Fixed Width.collection
Fun.collection
Japanese.collection
Korean.collection
Modern.collection
PDF.collection
Web.collection
"com.apple.Favorites.collection" and "com.apple.Recents.collection" are the two most common collections that can cause problems.
Another problem that usually results in Font Book quitting or crashing immediately on launch is caused by the fact that it's missing its "MainMenu.nib" interface file. If you open up the Console application (in the /Applications/Utilities/ folder) and look as you launch Font Book it should say something similar to the following:
<pre>2007-01-08 19:55:48 Font Book[3075] Unable to load nib file: MainMenu, exiting
2007-01-08 19:55:48 Font Book[3075]
* +[NSFileManager closeCatalog]:
selector not recognized
2007-01-08 19:55:48 Font Book[3075] An uncaught exception was raised</pre>
To fix this problem, you'll need to get
Pacifist to re-install the Font Book application from your OS X Tiger install disc. Then, you should download the latest version of the OS X Combo Update (currently OS X 10.4.8 Combo) and re-apply that over top of your existing OS X installation.
If your issue doesn't seem to be either of these, please post the contents of the crash log here...
Hope this helps....
Dual 2.7GHz PowerPC G5 w/ 2.5 GB RAM; 17" MacBook Pro w/ 2 GB RAM - Mac OS X (10.4.8)