This is asking for the number of combinations in which you can pick 8 books out of a group of 13.
The general formula for finding combinations is:
[tex] \left(\begin{array}{ccc}n\\k\end{array}\right) = \frac{n!}{k! (n-k)!} [/tex]
For this example where n = 13, k = 8
[tex] \frac{13!}{8! *5!} = \frac{13*12*11*10*9}{5*4*3*2*1} = 13*11*9 = 1287 [/tex]