In Font Book, you may come across a situation where the "User" collection in the Collection column appears dimmed with the word "Off" next to it, and you can't enable "User" from the Edit menu. This can happen after reinstalling Mac OS X 10.4 Tiger, or after manually removing items from the Fonts folder within your home folder's Library folder.
To resolve this issue, manually add a font to your Fonts folder (~/Library/Fonts), then restart your computer. The User collection should then be available in Font Book.