This is a whitepaper that goes into detail on the functionality and usage of nmap for portscanning.
You can download it from the following link: https://packetstormsecurity.com/files/download/129397/nmapps-approach.txt
Source: https://packetstormsecurity.com/files/129397/Nmap-Port-Scanning-A-Practical-Approach-Modified-For-Better.html