I recently changed the Copyright Year field in Bookcase to allow for multiple values, say for short story anthologies, etc. But one of the other things I had added was a 4-digit validator for the text edit book entry widget. Needless to say, limiting the entry to 4 digits prevents having multiple years separated by the semi-colon. The backend allowed it, but the entry widget didn't.
If you want to fix the problem, you can use this diff file, 0.5.2a-bcuniteditwidget.cpp.diff. WIthin the top-level bookcase-0.5.2a directory, run patch by the command:
patch -p0 < 0.5.2a-bcuniteditwidget.cpp.diff
and then recompile.
Or you can just wait for the next release.