Harish-Chandra pairs and their use in localization is discussed in Beilinson-Bernstein <i>A proof of Jantzen Conjectures</i>, available on [Joseph Bernstein's web page][1].  See in particular sections 1.8 and 3.3.

Other sources:

 1. Beilinson-Drinfeld, <i>Quantization of Hitchin's Integrable System and Hecke Eigensheaves</i>, sections 7.7-7.9.  Available [on line][2].
 2. Beilinson-Feigin-Mazur <i>Notes on Conformal Field Theory (Incomplete)</i>.  The discussion is centered around groupoids and the Virasoro algebra.  This is on [Barry Mazur's web page][3] (link on the side to "older material").


  [1]: http://www.math.tau.ac.il/~bernstei/Publication_list/Publication_list.html
  [2]: http://www.math.harvard.edu/~gaitsgde/grad_2009/
  [3]: http://www.math.harvard.edu/~mazur/index.html