After being asked a couple of times where the official documentation for Oracle E-Business Suite R12 is, note that R12.0.4 documentation set can be found at http://download.oracle.com/docs/cd/B40089_09/current/html/docset.html

[Note that the UK-specific HRMS documentation is included at this location, no need to go elsewhere for this.]

If you are interested in an overview of E-Business Suite, then read the Oracle Applications Concepts guide (part number B31450-04). In particular chapter one gives a useful grounding. It discusses topics such as the desktop tier, the application tier, web services, form services, business intelligence and the technology layer (explaining terms such as AK, AU, FND etc.)