Answer
$9.6\;\rm N$
Work Step by Step
As we can see in the figure below, there are two static friction forces exerted on the book upward and two normal forces with the same magnitude but opposing each other.
The book is on the verge to slide down, so the friction force must be up, as we drew below.
$$\sum F_y=2f_s-F_G=0$$
Thus, the maximum weight of a book that can be held by applying these two normal forces is given by
$$F_G=2f_s=2\mu_sF_n=2\cdot 0.8\cdot 6$$
$$F_G=\color{magenta}{\bf 9.6}\;\rm N$$
