1. Software requirements: Objects, Functions, and States;Davis,1993
2. Requirements Engineering: a Good Practice Guide;Sommerville,1997
3. Requirements Engineering: Processes and Techniques;Kontonya,1998
4. User involvement: key to success;Clavadetscher;IEEE Software, Requirements Engineering Issue,1998
5. The Standish Group, (1995): The High Cost of Chaos: http://www.standishgroup.com.