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>, section 7.9.  Available on line (e.g., Dennis Gaitsgory's web page)
 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.

  [1]: http://www.math.tau.ac.il/~bernstei/Publication_list/Publication_list.html