If you go to ‘http://publib.boulder.ibm.com/infocenter/iseries/v5r4/index.jsp’
and search on ‘DB2 for i5/OS SQL Reference’ select the first result in the left pane, and on the resulting page navigate down to ‘Printable PDF’s, you can download the reference manual that you will find most helpful. Some 1300 pages of good stuff.
As far as CURRENT goes, you’d have to be more specific. CURRENT is used in the names of several special registers such as CURRENT DATE, CURRENT DEBUG MODE, CURRENT SERVER, etc. It is also used in other places such as the WHERE CURRENT clause during cursor processing.
Beyond that, feel free to ask any specific questions. I’m sure there are many greater experts on the topic than I that visit this forum regularly, but I do work with it all the time and will be happy to help.