Nersesian, Roy L. (2010): Energy for the 21st Century: A Comprehensive Guide to Conventional and Alternative Sources, 2nd Edition, Armonk: M. E. Sharpe, Inc., pp. 92-122. (31 pages)

World Coal Institute. (2009). The Coal Resource: A Comprehensive Overview of Coal, London: World Coal Institute, pp. 2-41. www.worldcoal.org/bin/pdf/original_pdf_file/coal_resource_overview_of_coal_report(03_06_2009).pdf (40 pages)

U.S. Energy Information Administration. (2016). International Energy Outlook 2016. Washington: U.S. Energy Information Administration, p. 61-80. https://www.eia.gov/outlooks/ieo/pdf/0484(2016).pdf (20 pages)