Re: the hardest Kakuro puzzle?
I have done many puzzles in the past in the krazydad page (kakuros, sudokus, ...) I do not think you will find here the most difficult kakuro, in general the level in krazydad is medium, there is not any "crazy" puzzle there.
My experience with kakuro books (though I do not know that specific one from Amazon) is that you can find difficult and even very difficult kakuros but not "the most difficult".
However, sometimes I have seen really "crazy" kakuros somewhere in the web (I cann't remember in this moment where) and always the "crosses" are very wide, this increments the difficulty while the easier kakuros are obtained when the crosses or intersections are "slim", that is, a row againts a column, or two rows against two columns, so few cells in the intersection. With an intersection of 4 or 5 rows versus 4 or 5 columns the kakuro becomes almost unsolvable.