There is no doubt that Collins promoted O'Duffy while DeValera quickly replaced him on his ascent to Government.
Michael, another excellent study of Collins is "Mick" by Peter Hart, "Michael Collins'Intelligence War" by Michael T. Foy and you can add "The Big Fellow" by Frank O'Connor. (And, there are more)

That little lot, combined of course with Fiolar's biblography should keep you in reading matter and study for the rest of 2009 :-)