The Ballad of Halo Jones was originally intended to be 7 books (related I think to the seven ages of man) and Alan Moore had planned the basic plot of the entire series. Unfortunately he had a falling out with 2000AD over the ownership of the character and refused to write the remaining parts.

This is unfortunate and I am sure it would have been a very interesting journey to find out where Alan and Ian were going to take Halo. However at least we have the first three very enjoyable volumes.