I found this problem in Shiryaev's Problems in probability (Problem 3.4.14).
Let $\xi_1, \xi_2, \dots$ be a sequence of independent and $N(0, 1)$-distributed random variables. Setting $S_n = \xi_1 + \dots + \xi_n, \;n \ge 1$, find the limiting probability distribution (as $n \rightarrow \infty $) of the random variables $$\frac1n \sum_{k= 2}^{n}|S_{k - 1}|(\xi_k^2 - 1), \quad n \ge 2.$$
Could anyone give me a hint for solving this? Thank you!