He is now almost completely obscure; none of the standard English-language reference works on the history of mathematics I consulted so much as mentioned him, though doubtless the more ample histories do. The most accessible source for Killing's life and work is A. J. Coleman (1989), ``The Greatest Mathematical Paper of All Time,'' Mathematical Intelligencer vol. 11, no. 3, pp. 29--38. Coleman himself refers to papers by Thomas Hawkins, ``Non-euclidean Geometry and Weierstrassian Mathematics: The Background to Killing's work on Lie Algebras,'' Historica Mathematica 7 (1980) 289--342 and ``Wilhelm Killing and the Structure of Lie Algebras,'' Archive for Hist. Exact. Sc. 26 (1982) 126--192. I have not yet had the chance to read these.