8. Distribute 16 identical books to 4 classes, with each class getting at least one book, and the number of books each class receives being different. The number of different distribution methods is $\qquad$ .(Answer with a number)
Assume $x_{1}<x_{2}<x_{3}<x_{4}$, then the positive integer solutions for $x_{1}+x_{2}+x_{3}+x_{4}=16$ are $(1,2,3,10),(1,2,4,9),(1,2,5,8)$, $(1,2,6,7),(1,3,4,8),(1,3,5,7),(1,4,5,6),(2,3,4,7),(2,3,5,6)$, a total of 9 groups. Therefore, the number of different distribution methods is $9 A_{4}^{4}=216$.