PENN, Helen. Quality in early childhood services: an international perspective. Maidenhead: McGraw-Hill, Open University Press, 2011. ISBN 9780335240241. Dostupné také z: http://search.ebscohost.com/login.aspx?direct=true&scope=site&db=nlebk&AN=363056&authtype=ip,shib&custid=s3936755.