Quite a lot seem to be written on cohomology of groups like SL(n, Z), which I guess is an example of an arithmetic group - I have not linked to these papers on K-theory archive. There is something called Eisenstein cohomology of arithmetic groups. Some key names: Borel, Soulé.
PSPUM-9, in Algebraic groups folder.
Milne: Algebraic and arithmetic groups. In Alg gps folder
A few things are in the Arithmetic groups folder under NUMBER THEORY
Book by Borel? There are various introductions and surveys by Borel, see MathSciNet.
Lizhen Ji: Arithmetic groups and their generalizations
A survey by Borel which seems excellent: Armand Borel, Introduction to the cohomology of arithmetic groups (51–86) http://www.ams.org/mathscinet-getitem?mr=2272919
See also Cohomology of arithmetic groups
http://mathoverflow.net/questions/3701/stable-homology-of-arithmetic-groups
nLab page on Arithmetic group