Oracle 11i And R12 Documentation

11i Documentation for DBA's
http://download.oracle.com/docs/cd/B25516_18/current/html/docset.html

Books that can be read:-
Oracle Applications Installation Guide: Using Rapid Install
Oracle Applications Maintenance Procedures
Oracle Applications Maintenance Utilities
Oracle Applications Concepts
Oracle Applications System Administrator's Guide - Configuration (zipped)

R12 Documentation for DBA's
http://download.oracle.com/docs/cd/B40089_10/current/html/docset.html

Books that can be read:-
Oracle Applications Installation Guide: Using Rapid Install
Oracle Applications Maintenance Procedures
Oracle Applications Maintenance Utilities
Oracle Applications Patching Procedures
Oracle Applications Concepts
Oracle Applications System Administrator's Guide - Configuration
Oracle Applications System Administrator's Guide - Maintenance
Oracle Applications System Administrator's Guide - Security

Put in your doubts in the comments section, I will surely answer them.