ASTM Standard D143, 2009, "Standard Test Methods for Small Clear Specimens of Timber," ASTM International, West Conshohocken, PA, 2009, www.astm.org. 31 p.
ASTM Standard E10, 2012, "Standard Test Method for Brinell Hardness of Metallic Materials," ASTM International, West Conshohocken, PA, 2012, www.astm.org. 32 p.
ASTM Standard D7136, 2012. "Standard Test Method for Measuring the Damage Resistance of a Fiber-Reinforced Polymer Matrix Composite to a Drop-Weight Impact Event," ASTM International, West Conshohocken, PA, 2012, www.astm.org. 16 p.
Bektas, I., M.H. Alma, and N. As. 2001. Determination of the relationships between Brinell and Janka hardness of eastern beech (Fagus orientalis Lipsky). Forest Products Journal 51(11/12):84-88.
Forest Products Laboratory. 1999, "Wood Handbook-Wood as an Engineering Material," Gen. Tech. Rep. FPL-GTR-113. Madison, WI: USDA, Forest Service, Forest Products Laboratory. 463 p.
Niemiec, S., G.R. Ahrens, S. Willits, and D.E. Hibbs. 1995. Hardwoods of the Pacific Northwest, Research Contribution 8. Forest Research Laboratory, Oregon State University.
And more casual but interesting stories and looks at Janka:
Article from Makezine.com: How Wood Hardness is Measured