You can buy a Kindle edition of a collection of The Dignified Rant posts on the theme of privatized warfare (including mercenaries):
The Dignified Rant On: Private Warfare (No. 1 January 2012) is a collection of posts from my blog, The Dignified Rant, on the issue of mercenaries, privatized warfare, and nonstate actors emerging on the state-centric world stage.
This planned series has been more work than it justifies. I did start another on Iran's nuclear issue but never finished it. I don't rule out doing another one, perhaps in a different style. But I don't have anything on the horizon.
I do intend to add some titles here not based on the blog, in time.