Cloud Security and Governance: Who’s on your Cloud?by Sumner Blount and Rob Zanella
Securing Cloud Services: A pragmatic approach to security architecture in the Cloud by Lee Newcombe
Cloud Computing: Assessing the risks by Jared Carstensen, JP Morgenthal and Bernard Golden
Above the Clouds: Managing Risk in the World of Cloud Computing by Kevin T McDonald
From a legal perspective: Cloud Computing a practical introduction to the legal issues, by Renzo Marchini
See also the Cloud Security Alliance or CSA (www.cloudsecurityalliance.org) which provides a security framework developed with cloud considerations in mind - the Cloud Controls Matrix (CCM).