The are all long books, around 15 hours, with complex plots and none trivial characters. I have rated all six of these books 5 stars, my highest rating, because I often forget where I am driving to and don't want to turn the book off when I do get there.
As I recall the first book (By Order of the President) stands alone while the second and third continue the story. The Outlaws is book VI in the Presidential Agent series; it pretty much recapitulates the earlier books because the detailed history is required to follow the current action. Ofter the review of previous story point is tedious but I really enjoyed the way it was integrated into the story (because it was like going back to old friend and reminiscing.)
Book I, By Order of the President (2005) Book II, The Hostage (2006) Book III, The Hunters (2007) Book IV, The Shooters (2008) Book V, Black Ops (2009) Book VI, The Outlaws (2010)