New bill books

HELLO FRIENDS, CAN ANYBODY TELL ME IS IT NECESSARY TO HAVE A NEW BILL BOOK IN FINANCIAL YEAR OR OLD BOOK MAY CONTINUE.

SUPPOSE BILL BOOK NO 300-400 AND IT ENDS ON BILL NO 316 ON 31/03/2013 AND REMAINING BILLS ARE THERE.

NOW ON 01/04/2013 I HAVE TO START A NEW BILL BOOK FROM 401 OR CAN I CONTINUE THAT OLD BOOK.

PLS. HELP ME.

THANKS.