Kazhdan and Lusztig classified the simple modules of an affine Hecke algebra H
q
(
E C*) provided that
is not a root of 1 (Invent. Math. 1987). Ginzburg had some very interesting work on affine Hecke algebras. Combining these results simple H
-modules can be classified provided that the order of
is not too small. These Lecture Notes of N. Xi show that the classification of simple H
-modules is essentially different from general cases when
is a root of 1 of certain orders. In addition the based rings of affine Weyl groups are shown to be of interest in understanding irreducible representations of affine Hecke algebras. Basic knowledge of abstract algebra is enough to read one third of the book. Some knowledge of K-theory, algebraic group, and Kazhdan-Lusztig cell of Cexeter group is useful for the rest