Calculating coefficient of resitution

[JizzaDaMan]

I've been told that the coefficient of restitution is a little like friction; it's a measure of how elastic a collision is between two objects, it's not an individual property of each object itself.

Now let's say you have two balls bouncing on the ground. Ball 1 has a coefficient of restitution with the ground of 0.8, and ball 2 has a coefficient of 0.7 between itself and the ground.

This way you have the ground as a common reference. Is it then acceptable to assign a coefficient of restitution to each ball as an individual property?

Given this, is there then a way to calculate the coefficient of restitution in a collision between the two balls?

 

[CWatters]

http://physics.umd.edu/lecdem/services/refs_scanned_WIP/3%20-%20Vinit's%20LECDEM/C716/1/GetPDFServlet.pdf

Abstract

A perfectly happy ball is one that bounces to its original height when dropped on a massive, rigid
surface. A completely unhappy ball does not bounce at all. In the former case, the coefficient of
restitution COR is unity. In the latter case, the COR is zero. It is shown that when an unhappy ball
collides with a happy ball, the COR increases from zero to unity as the stiffness of the happy ball
decreases from infinity to zero...

 

[JizzaDaMan]

I don't quite understand this phrase: "the COR increases from zero to unity as the stiffness of the happy ball
decreases from infinity to zero." or indeed it's implications.