Founder, you mention study books so the following real world answer may or may not help if it just exams your trying to pass.
Taking the B757/767 as an example.
An index unit ( IU ) is just another way of expressing a moment.
So remember a moment is the mass X distance of mass from some point.
The manufacturer specifies the "some point" as a datum point usually somewhere forward of the fuselage nose. The distance from it is usually in inches.
The DOW ( aircraft ready for service without fuel) is simply a sum of all the moments for catering, crew etc.
However with airliners the numbers can start to get very big and the C of G calculations can therefore become very user unfriendly.
So operators use index numbers (IU,s) with their own trim sheets to simplify the procedure.
On the B767 an IU of zero equals 20%MAC ( which sits about in the middle of the loadsheet). If it helps 20 %MAC is actually 972.6 inches back from the B767 datum point.
So any positive IU numbers make the aircraft more tail heavy and negatives more nose heavy. A typical DOI on the B757 would be +6
so IU equals : mass X (arm - 972.6)
They then divide the whole thing by 50000 to get the IU down to single numbers.