I have problem with Font Book crashing when opened.
Problem appeared somewhere after I updated my iMac to OS X Maverick - but I cannot really say is this the cause or not.
When opened, FontBook shows the main window and the error report "Multiple copies of this font are installed" (even when i have removed all the fonts from Library -> Fonts folder). Then it freezes and after a minute or so shuts down with crash report.
I checked forums but havent yet find any solutions for Maverick.