Who is Henry Cook from Gideon's Trumpet and what is their importance?

Cook is the man who testified in court that he saw Gideon inside the poolroom during the theft of liquor, cigarettes and change. Cook's own reasons for being on the street at five-thirty in the morning were not thoroughly questioned at the time of the initial trial but later became an issue. Cook's statement that he saw Gideon inside the poolroom also came under fire at the retrial when Gideon's attorney pointed out that the advertisements on the windows would have blocked the view.