the chris Harrison article (above) does have better references. here they are
References
Blair, Claude (1958). European Armor, circa 1066 to circa 1700. London: B.T. Batsford Ltd.
Bradbury, Jim (2004). The Routledge Companion to Medieval Warfare. London: Routledge, Taylor and Francis Group.
Carman, John & Harding, Anthony (1999). Ancient Warfare, Archeological Perspectives. Gloucestershire, UK: Tempus Publishing Limited.
Connolly, Peter (1981). Greece and Rome at War. London: Macdonald.
Demmin, Auguste (1964) Die Kriegswaffen im ihren Geschichtlichen Entwickelungen von den Ältesten Zeiten bis auf den Gegenwart. (Weapons of War and their Historica Developments from the Past to the Present) Leipzig: Gg Olms.
DeVries, Kelly (1956). Medieval Military Technology. Lewiston, NY: Broadview Press Ltd.
Ferrill, Arther (1985). The Origins of War, From the Stone Age to Alexander the Great. London: Thames and Hudson Ltd.
Gabriel, Richard & Metz, Karen (1991). From Sumer to Rome: The Military Capabilities of Ancient Armies. Westport, CT: Greenwood Publishing Group.
Grunfeld, Foster (1996). The Unsung Sling. Military History Quarterly, V9 #1. p. 51-55.
Hawkins, Walter (1847). Observations on the Use of the Sling, as a Warlike Weapon Among the Ancients. London: J.B. Nichols and Son.
Hogg, O.F.G. (1968). Clubs to Cannon: Warfare and Weapons Before the Introduction of Gunpowder. London: Gerald and Company, Ltd.
Lindblom, K.G. (1940). The Sling, Especially in Africa. Stockholm: Staten Etnografsika Museum.
Korfmann, Manfred (1973). The Sling as a weapon. Scientific American, October 229(4), p. 35-42.
Martin, Paul (1968). Arms and Armor, From the 9th to the 17th Century. Rutland, Vermont: Charles E. Tuttle Co., Inc.
Nicholson, Helen (2004). Medieval Warfare, Theory and Practice of War in Europe 300-1550. New York: Palgrave Macmillan.
Norris McWhirter, ed. (1985). The Guinness Book of World Records, 23rd US edition. New York: Sterling Publishing Co., Inc.
Richardson, Thom (1998a). Ballistic Testing of Historical Weapons. Royal Armouries Yearbook 3, p. 50-52.
Richardson, Thom (1998b). Ballistic Testing of the Sling. Royal Armouries Yearbook 3, p. 44-49.
Skobelev, D. A. (2000). Sling: Projectiles and the Methods of Throwing in the Antiquity (title translated from Russian). Para Bellvm. March 23, 2005.
http://www.xlegio.ru/pubs/skobelev/sling2.htm
Snodgrass, A. M. (1967). Arms and Armor of The Greeks. Ithaca, New York: Cornell University Press.
Underwood, Richard (1999). Anglo-Saxon Weapons and Warfare. Gloucestershire, UK: Tempus Publishing Limited. The Sling in Medieval Europe 10
Walker, R. (2004). Density of Materials: Bulk Materials. March 3rd, 2005.
http://www.simetric.co.uk/si_materials.htm
Wise, Terence (1976). Medieval Warfare. New York: Hastings House.
Wise, Terence (1980). The Conquistadores. Oxford: Osprey Publishing
While I can't say they are top references, as I really don't know, they appear to have some legitimacy.