Appalling mercy: For this phrase I am indebted to Perelandra , by C. S. Lewis (though Lewis's phrase is actually "appalling love"). Perelandra builds an enchanting world, and sets within it a gripping, desperate conflict between good and evil that elucidates the nature of each. I found it one of the most disturbing books I have ever read (even more so than its sequel, That Hideous Strength ), but every re-read is a delight. Lewis's insight is truly impressive, as is his ability to make theological truths relevant – no, to show us how desperately relevant they are – to our daily lives, whether on Perelandra or Earth.
I recommend reading the whole series ( Out of the Silent Planet, Perelandra, and That Hideous Strength) in order, and letting it explain itself. However, a good plot summary/review of Perelandra, with some spoilers, can be found here.