what does it mean to leave something "on the books"
i.e. Germany left several of the less severe Nazi laws regarding 'Gypsies' on the books.
Laws are passed and written in law books as 'statutes'. When a law is repealed, then obviously, it is removed from the books (the written account of the current laws of the land). So, after the war, laws which had been enacted by the Nazis were repealed - but not all: some were left 'on the books'. That is, they were still in operation.