Why is the introduction for "Green Ghost" so long compared to other introductions?
This is an excellent question, and I wish I knew the answer. For some reason, Robert Arthur got long-winded in the introduction to "Green Ghost" even though the boys were already introduced to us in great detail in the first 3 stories. Of course, part of it may be that he wrote the first three as a block (i.e., it's generally believed and accepted that Arthur took the first three stories to Random House when he tried to sell them idea of the Three Investigators series); so there would have been a time lag between the first three stories and "Green Ghost" and he may have thought it necessary to provide a longer introduction.
Interestingly, the introduction to "Green Ghost" in the Scholastic paperback version (the original paperback version with the Kane cover art) is different than the introduction provided in the hardback version of that book. You can go to my friend Robert Held's web site at tunneltwo.com to read the difference in the introductions by clicking here.