When do you see this error? What are you trying to do when this error occurs?
The error code -54 is a permissions error (Software lock on file; Not a subscriber [permissions error on file open])
So the first thing I'd try would be to repair the permissions on the drive, where your iTunes Library is located.
If the library is on an external volume, you can use Disk Utility, but to repair your system volume, boot into Recovery and use the Utilities menu (Disk Utility) there. OS X Lion: About Lion Recovery