Basically,
Miracles presents arguments against naturalism, which claims that miracles are intrinsically impossible. Lewis argues that there is no reason to think that God cannot or does not act directly in our world.
On what basis do you want to compare it to
The Problem of Pain and
Mere Christianity? I think
Miracles requires a little more effort from the reader than MC to follow the arguments--is this the type of comparison you want?
The best way to find out about the book, of course, is to read it!