From the United Nations Office on Drugs and Crime:

Cocaine Trafficking in West Africa: The threat to stability and development
(with special reference to Guinea-Bissau) - December 2007 (pdf)
Drug Trafficking as a Security Threat in West Africa - November 2008 (pdf)
Transnational Trafficking and the Rule of Law in West Africa: A Threat Assessment - July 2009 (pdf)

The Pulitzer Center on Crisis Reporting has done a good series:

Guinea-Bissau: West Africa's New Achilles' Heel
Untold Stories