Mercenary
ISBN 1855327635
Goldsworthy, Adrian (2006). The Fall of Carthage: The Punic Wars 265–146 BC. London: Phoenix. ISBN 978-0-304-36642-2.
Milliard, Todd S.; Overcoming post-colonial myopia: A call to recognize and regulate private military companies Archived 1 December 2012 at the Wayback Machine(PDF), in Military Law Review Vol 173, June 2003. At the time of publication Major Milliard was a Judge Advocate in the United States Army Judge Advocate General's Corps
Anthony Mockler, Storia dei mercenari: Da Senofonte all'Iraq. Odoya, 2012. ISBN 978-8862881531.
Schuster, Carl Otis (2016). "Memnon and Mentor of Rhodes (ca. 380–333, ca. 385–340)". In Phang, Sara E.; Spence, Iain; Kelly, Douglas; Londey, Peter (eds.). Conflict in Ancient Greece and Rome: The Definitive Political, Social, and Military Encyclopedia. ABC-CLIO. ISBN 978-1610690201.

Further reading: 

Historical: Atwood, Rodney. The Hessians: Mercenaries from Hessen-Kassel in the American Revolution (Cambridge University Press, 1980).
Avant, Deborah. "From mercenary to citizen armies: Explaining change in the practice of war." International Organization (2000): 41–72. online
Fetter, Frank Whitson. “Who Were the Foreign Mercenaries of the Declaration of Independence?” Pennsylvania Magazine of History and Biography, vol. 104, no. 4, 1980, pp. 508–513. online
Ingrao, Charles. "" Barbarous Strangers": Hessian State and Society during the American Revolution." American Historical Review 87.4 (1982): 954–976 online
Ingrao, Charles W. The Hessian mercenary state: ideas, institutions, and reform under Frederick II, 1760–1785 (Cambridge University Press, 2003).
Niccolò Machiavelli. The Prince. 1532. Ch. 12.
"Military science in western Europe in the sixteenth century." Prologue: The nature of armies in the 16th century
Mockler, Anthony. The Mercenaries: The Men Who Fight for Profit – from the Free Companies of Feudal France to the White Adventurers in the Congo. Macmillan, 1969.
Percy, Sarah. Mercenaries: The history of a norm in international relations (Oxford University Press, 2007).
Schmidt, H. D. "The Hessian Mercenaries: The Career of a Political Cliché." History 43.149 (1958): 207–212 online
Scullard, Howard H. (2006) [1989]. "Carthage and Rome". In Walbank, F. W.; Astin, A. E.; Frederiksen, M. W.; Ogilvie, R. M. (eds.). Cambridge Ancient History: Volume 7, Part 2 (2nd ed.). Cambridge: Cambridge University Press. pp. 486–569. ISBN 978-0-521-23446-7.
Thomson, Janice E. Mercenaries, pirates, and sovereigns: state-building and extraterritorial violence in early modern Europe. Princeton University Press, 1994. ISBN 1400808014 Describes the building of the modern state system through the states' "monopolization of extraterritorial violence."
Underwood, Matthew. "Jealousies of a standing army: the use of mercenaries in the American revolution and its implications for Congress's role in regulating private military firms." 'Northwestern University Law Review 106 (2012): 317-349.

Since 1970s: Guy Arnold. Mercenaries: The Scourge of the Third World. Palgrave Macmillan, 1999. ISBN 978-0312222031
Doug Brooks & Shawn Lee Rathgeber. "The Industry Role in Regulating Private Security Companies", Canadian Consortium on Human Security – Security Privatization: Challenges and Opportunities, Vol. 6.3, University of British Columbia, March 2008.
Anthony Mockler. Hired Guns and Coups d'Etat: Mercenaries: Thirty Years 1976–2006. Hunter Mackay, 2007.
Anthony Mockler. The New Mercenaries: The History of the Mercenary from the Congo to the Seychelles. Paragon House, 1987.
Robert Young Pelton. Hunter Hammer and Heaven, Journeys to Three World's Gone Mad, ISBN 1585744166
Jeremy Scahill. Blackwater: The Rise of the World's Most Powerful Mercenary Army, Nation Books, 2007. ISBN 1560259795
Peter J. Woolley. "Soldiers of Fortune," The Common Review, v. 5, no. 4(2007), pp. 46–48. Review essay.

Status in international law: Marina Mancini; Private Military and Security Company Employees: Are They the Mercenaries of the Twenty-first Century?, EUI Working Paper AEL 2010/5, European University Institute, San Domenico di Fiesole, 2010, ISSN 1831-4066.
Katherine Fallah; Corporate Actors: the Legal Status of Mercenaries in Armed Conflict, International Review of the Red Cross, (2006)
Eliav Lieblich; "The Status of mercenaries in International Armed Conflict as a case of politicization of International Humanitarian Law", Bucerius Law Journal, (2009)

Private military companies (PMCs): Robert Young Pelton; Licensed to Kill: Hired Guns in the War on Terror, Crown, (2006), ISBN 1400097819
José Alvear Restrepo Lawyers' Collective; Private Security Transnational Enterprises in Colombia Archived 17 April 2008 at the Wayback Machine February 2008

External links: The Security Contracting Network is resource and community of security contracting professionals.
PMCs Monitor: An international organization which advocates for tighter rules
The Working Group on the use of mercenaries as a means of violating human rights and impeding the exercise of the right of peoples to self-determination UNHCR