NCSC-TG-024 Vol. 1/4: A Guide to Procurement of Trusted Systems: An Introduction to Procurement Initiators on Computer Security Requirements, December 1992. (Purple Book)
You can download it from the following link: https://packetstormsecurity.com/files/download/14014/NCSC-TG-024-1.html
Source: https://packetstormsecurity.com/files/14014/NCSC-TG-024-1.html.html