The National Security Agency recently released a new document titled, "An Overview of Cloud Computing," which, as its title suggests, is an overview of cloud computing. You can read here.
The 13-page paper is a rather easy to read introduction to the concept of cloud computing even if it does take some liberties with the expertise of its assumed- readersship. However, this is a not an insurmountable challenge. The main disappointment with the paper is that it is solely a product of open- source research and insight. There is nothing here that is NSA specific or NSA-unique. Anyone with the appropriate knowledge and willingness to create such a report could do so.