We can also use a new operator called the combined comparison operator to compare two Ruby objects. The combined comparison operator looks like this:
<=>. It returns
0 if the first operand (item to be compared) equals the second,
1 if the first operand is greater than the second, and
-1 if the first operand is less than the second.
A block that is passed into the
sort method must return either
-1. It should return -1 if the first block parameter should come before the second,
1 if vice versa, and
0 if they are of equal weight, meaning one does not come before the other (i.e. if two values are equal)
Use the combined comparison operator to compare
book_2 (in that order). Before you run the code, think about what the result will be.