George Lusztig 
Representations of Finite Chevalley Groups [PDF ebook] 

The notes came from a series of lectures given by the author at a CBMS Regional Conference held at Madison, Wisconsin, in August 1977. The main purpose of the notes was to show how $l$-adic cohomology of algebraic varieties over fields of positive characteristic can be used to obtain information on the representations of finite Chevalley groups.