Answer
$210$
Work Step by Step
If we want to choose $k$ elements out of $n$ disregarding the order, not allowing repetition, we can do this in $_{n}C_k=\frac{n!}{(n-k)!k!}$ ways.
The order doesn't matter here in the set of books, thus we have to use combinations. Thus $_{10}C_{4}=\frac{10!}{(10-4)!4!}=210$
